Verified on natty:

(natty-amd64)root@clint-MacBookPro:/tmp# apt-get install mercurial-git
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  python-dulwich python-fastimport
Suggested packages:
  git-core bzr-fastimport
The following NEW packages will be installed:
  mercurial-git python-dulwich python-fastimport
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 193 kB of archives.
After this operation, 1196 kB of additional disk space will be used.
Do you want to continue [Y/n]? 
Get:1 http://127.0.0.1/ubuntu/ natty/universe python-dulwich amd64 0.7.0-1 [145 kB]
Get:2 http://127.0.0.1/ubuntu/ natty/universe mercurial-git all 0.2.3-1 [19.2 kB]
Get:3 http://127.0.0.1/ubuntu/ natty/universe python-fastimport all 0.9.0~bzr298-1 [29.1 kB]
Fetched 193 kB in 1s (120 kB/s)             
Selecting previously deselected package python-dulwich.
(Reading database ... 16631 files and directories currently installed.)
Unpacking python-dulwich (from .../python-dulwich_0.7.0-1_amd64.deb) ...
Selecting previously deselected package mercurial-git.
Unpacking mercurial-git (from .../mercurial-git_0.2.3-1_all.deb) ...
Selecting previously deselected package python-fastimport.
Unpacking python-fastimport (from .../python-fastimport_0.9.0~bzr298-1_all.deb) ...
Setting up python-dulwich (0.7.0-1) ...
Setting up mercurial-git (0.2.3-1) ...
Setting up python-fastimport (0.9.0~bzr298-1) ...
Processing triggers for python-support ...
(natty-amd64)root@clint-MacBookPro:/tmp# hg clone git://git.debian.org/git/pkg-kde/kde-std/calligra.git
destination directory: calligra
importing Hg objects into Git
Counting objects: 1500, done.
Compressing objects: 100% (603/603), done.
Total 1500 (delta 575), reused 1500 (delta 575)
importing Git objects into Hg
** unknown exception encountered, please report by visiting
**  http://mercurial.selenic.com/wiki/BugTracker
** Python 2.7.1+ (r271:86832, Apr 11 2011, 18:13:53) [GCC 4.5.2]
** Mercurial Distributed SCM (version 1.7.5)
** Extensions loaded: bookmarks, git
Traceback (most recent call last):
  File "/usr/bin/hg", line 38, in <module>
    mercurial.dispatch.run()
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 16, in run
    sys.exit(dispatch(sys.argv[1:]))
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 36, in dispatch
    return _runcatch(u, args)
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 58, in _runcatch
    return _dispatch(ui, args)
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 593, in _dispatch
    cmdpats, cmdoptions)
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 401, in runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 644, in _runcommand
    return checkargs()
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 598, in checkargs
    return cmdfunc()
  File "/usr/lib/pymodules/python2.7/mercurial/dispatch.py", line 591, in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
  File "/usr/lib/pymodules/python2.7/mercurial/util.py", line 426, in check
    return func(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/mercurial/commands.py", line 732, in clone
    branch=opts.get('branch'))
  File "/usr/lib/pymodules/python2.7/mercurial/hg.py", line 337, in clone
    dest_repo.clone(src_repo, heads=revs, stream=stream)
  File "/usr/lib/pymodules/python2.7/mercurial/localrepo.py", line 1886, in clone
    return self.pull(remote, heads)
  File "/usr/lib/pymodules/python2.7/hgext/git/hgrepo.py", line 12, in pull
    git.fetch(remote.path, heads)
  File "/usr/lib/pymodules/python2.7/hgext/git/git_handler.py", line 100, in fetch
    self.import_git_objects(remote_name, refs)
  File "/usr/lib/pymodules/python2.7/hgext/git/git_handler.py", line 407, in import_git_objects
    self.import_git_commit(commit)
  File "/usr/lib/pymodules/python2.7/hgext/git/git_handler.py", line 414, in import_git_commit
    hg_branch, extra) = self.extract_hg_metadata(commit.message)
  File "/usr/lib/pymodules/python2.7/hgext/git/git_handler.py", line 791, in extract_hg_metadata
    command, data = line.split(" : ", 1)
ValueError: need more than 1 value to unpack
(natty-amd64)root@clint-MacBookPro:/tmp# vi /etc/apt/sources.list
(natty-amd64)root@clint-MacBookPro:/tmp# apt-get update
Ign http://127.0.0.1 natty InRelease
Ign http://127.0.0.1 natty-updates InRelease
Ign http://127.0.0.1 natty-proposed InRelease
Hit http://127.0.0.1 natty Release.gpg 
Ign http://security.ubuntu.com natty-security InRelease
Hit http://127.0.0.1 natty-updates Release.gpg
Get:1 http://security.ubuntu.com natty-security Release.gpg [198 B]
Get:2 http://security.ubuntu.com natty-security Release [39.8 kB]
Get:3 http://127.0.0.1 natty-proposed Release.gpg [198 B]
Hit http://127.0.0.1 natty Release                             
Hit http://127.0.0.1 natty-updates Release              
Get:4 http://127.0.0.1 natty-proposed Release [39.8 kB]                        
Ign http://127.0.0.1 natty/main TranslationIndex                  
Ign http://127.0.0.1 natty/multiverse TranslationIndex
Ign http://127.0.0.1 natty/restricted TranslationIndex
Ign http://127.0.0.1 natty/universe TranslationIndex
Ign http://127.0.0.1 natty-updates/main TranslationIndex
Ign http://127.0.0.1 natty-updates/multiverse TranslationIndex
Ign http://127.0.0.1 natty-updates/restricted TranslationIndex
Ign http://127.0.0.1 natty-updates/universe TranslationIndex
Get:5 http://security.ubuntu.com natty-security/main amd64 Packages [350 kB]
Ign http://127.0.0.1 natty-proposed/main TranslationIndex
Ign http://127.0.0.1 natty-proposed/multiverse TranslationIndex
Ign http://127.0.0.1 natty-proposed/restricted TranslationIndex
Get:6 http://security.ubuntu.com natty-security/restricted amd64 Packages [4790 B]
Get:7 http://security.ubuntu.com natty-security/universe amd64 Packages [96.6 kB]
Ign http://127.0.0.1 natty-proposed/universe TranslationIndex                  
Hit http://127.0.0.1 natty/main amd64 Packages            
Hit http://127.0.0.1 natty/restricted amd64 Packages      
Get:8 http://security.ubuntu.com natty-security/multiverse amd64 Packages [3533 B]
Ign http://security.ubuntu.com natty-security/main TranslationIndex        
Ign http://security.ubuntu.com natty-security/multiverse TranslationIndex
Ign http://security.ubuntu.com natty-security/restricted TranslationIndex
Ign http://security.ubuntu.com natty-security/universe TranslationIndex
Hit http://127.0.0.1 natty/universe amd64 Packages                    
Hit http://127.0.0.1 natty/multiverse amd64 Packages
Hit http://127.0.0.1 natty-updates/main amd64 Packages
Hit http://127.0.0.1 natty-updates/restricted amd64 Packages
Hit http://127.0.0.1 natty-updates/universe amd64 Packages
Hit http://127.0.0.1 natty-updates/multiverse amd64 Packages
Get:9 http://127.0.0.1 natty-proposed/main amd64 Packages [140 kB]
Get:10 http://127.0.0.1 natty-proposed/restricted amd64 Packages [20 B]
Ign http://security.ubuntu.com natty-security/main Translation-en     
Get:11 http://127.0.0.1 natty-proposed/universe amd64 Packages [32.4 kB]
Ign http://security.ubuntu.com natty-security/multiverse Translation-en
Ign http://security.ubuntu.com natty-security/restricted Translation-en
Ign http://security.ubuntu.com natty-security/universe Translation-en
Get:12 http://127.0.0.1 natty-proposed/multiverse amd64 Packages [1199 B]
Ign http://127.0.0.1 natty/main Translation-en                                                                       
Ign http://127.0.0.1 natty/multiverse Translation-en
Ign http://127.0.0.1 natty/restricted Translation-en
Ign http://127.0.0.1 natty/universe Translation-en
Ign http://127.0.0.1 natty-updates/main Translation-en
Ign http://127.0.0.1 natty-updates/multiverse Translation-en
Ign http://127.0.0.1 natty-updates/restricted Translation-en
Ign http://127.0.0.1 natty-updates/universe Translation-en
Ign http://127.0.0.1 natty-proposed/main Translation-en
Ign http://127.0.0.1 natty-proposed/multiverse Translation-en
Ign http://127.0.0.1 natty-proposed/restricted Translation-en
Ign http://127.0.0.1 natty-proposed/universe Translation-en
Fetched 709 kB in 14s (48.6 kB/s)
Reading package lists... Done
(natty-amd64)root@clint-MacBookPro:/tmp# apt-get install mercurial
Reading package lists... Done
Building dependency tree       
Reading state information... Done
mercurial is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 4 not upgraded.
(natty-amd64)root@clint-MacBookPro:/tmp# apt-get install mercurial-git
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
  mercurial-git
1 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
Need to get 19.4 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://127.0.0.1/ubuntu/ natty-proposed/universe mercurial-git all 0.2.3-1ubuntu0.1 [19.4 kB]
Fetched 19.4 kB in 0s (47.3 kB/s)       
(Reading database ... 16728 files and directories currently installed.)
Preparing to replace mercurial-git 0.2.3-1 (using .../mercurial-git_0.2.3-1ubuntu0.1_all.deb) ...
Unpacking replacement mercurial-git ...
Processing triggers for python-support ...
Setting up mercurial-git (0.2.3-1ubuntu0.1) ...
Processing triggers for python-support ...
(natty-amd64)root@clint-MacBookPro:/tmp# hg clone git://git.debian.org/git/pkg-kde/kde-std/calligra.git
destination directory: calligra
importing Hg objects into Git
Counting objects: 1500, done.
Compressing objects: 100% (603/603), done.
Total 1500 (delta 575), reused 1500 (delta 575)
importing Git objects into Hg
updating to branch default
93 files updated, 0 files merged, 0 files removed, 0 files unresolved
(natty-amd64)root@clint-MacBookPro:/tmp#