I got the same error as Guilhem:

What I did to get it was:
- Branch a tree (mysql-6.0)
- Merge with another tree (mysql-5.1-maria) with bzr merge --weave
- Fixed merge issues, did make and make test to verify tre
- bzr resolved --all
- bzr gcommit

I am now stuck and can't continue developing until this is fixed.
Please prioritize this bug accordingly (we have a support contract with Canonical)

bzr status output looks sane (it completes with no errors or warnings(

I am using bzr version 1.8 and gtk plugin 0.96.0dev1
I applied the gtk-fix.patch, but it didn't help; Now I get another error instead.

Here is the output from bzr gcommit after the patch was applied:

(/my/mysql-6.0-maria-tmp) bzr gcommit
bzr: ERROR: exceptions.AssertionError:

Traceback (most recent call last):
  File "/usr/local/lib/python2.5/site-packages/bzrlib/commands.py", line 849, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/usr/local/lib/python2.5/site-packages/bzrlib/commands.py", line 795, in run_bzr
    ret = run(*run_argv)
  File "/usr/local/lib/python2.5/site-packages/bzrlib/commands.py", line 495, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/monty/.bazaar/plugins/gtk/__init__.py", line 394, in run
    dlg = CommitDialog(wt)
  File "/home/monty/.bazaar/plugins/gtk/commit.py", line 121, in __init__
    self.fill_in_data()
  File "/home/monty/.bazaar/plugins/gtk/commit.py", line 135, in fill_in_data
    self._fill_in_files()
  File "/home/monty/.bazaar/plugins/gtk/commit.py", line 193, in _fill_in_files
    ) in iter_changes_to_status(self._basis_tree, self._wt):
  File "/home/monty/.bazaar/plugins/gtk/diff.py", line 659, in iter_changes_to_status
    assert kinds[0] is not None
AssertionError

bzr 1.8 on python 2.5.1 (linux2)
arguments: ['/usr/local/bin/bzr', 'gcommit']
encoding: 'ANSI_X3.4-1968', fsenc: 'ANSI_X3.4-1968', lang: 'C'
plugins:
  difftools            /home/monty/.bazaar/plugins/difftools [0.91.0]
  extmerge             /home/monty/.bazaar/plugins/extmerge [unknown]
  gtk                  /home/monty/.bazaar/plugins/gtk [0.96.0dev1]
  launchpad            /usr/local/lib/python2.5/site-packages/bzrlib/plugins/launchpad [unknown]
  multiparent          /usr/local/lib/python2.5/site-packages/bzrlib/plugins/multiparent.py [unknown]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.

Here is the associated output from .bzr.log

0.042  encoding stdout as sys.stdout encoding 'ANSI_X3.4-1968'
0.044  bzr arguments: [u'gcommit']
0.044  looking for plugins in /home/monty/.bazaar/plugins
0.234  importmbox not available unless bzrtools is installed
0.274  looking for plugins in /usr/local/lib/python2.5/site-packages/bzrlib/plugins
0.275  Plugin name __init__ already loaded
0.275  Plugin name gtk already loaded
0.275  Plugin name __init__ already loaded
0.278  looking for plugins in /usr/lib/python2.5/site-packages/bzrlib/plugins
0.308  encoding stdout as sys.stdout encoding 'ANSI_X3.4-1968'
1.138  opening working tree '/home/my/mysql-6.0-maria-tmp'
10.902  Traceback (most recent call last):
  File "/usr/local/lib/python2.5/site-packages/bzrlib/commands.py", line 849, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/usr/local/lib/python2.5/site-packages/bzrlib/commands.py", line 795, in run_bzr
    ret = run(*run_argv)
  File "/usr/local/lib/python2.5/site-packages/bzrlib/commands.py", line 495, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/monty/.bazaar/plugins/gtk/__init__.py", line 394, in run
    dlg = CommitDialog(wt)
  File "/home/monty/.bazaar/plugins/gtk/commit.py", line 121, in __init__
    self.fill_in_data()
  File "/home/monty/.bazaar/plugins/gtk/commit.py", line 135, in fill_in_data
    self._fill_in_files()
  File "/home/monty/.bazaar/plugins/gtk/commit.py", line 193, in _fill_in_files
    ) in iter_changes_to_status(self._basis_tree, self._wt):
  File "/home/monty/.bazaar/plugins/gtk/diff.py", line 659, in iter_changes_to_status
    assert kinds[0] is not None
AssertionError

10.906  return code 4