build: errors reported after pristine-tar introduction

[Nish Aravamudan]

Andreas reported in LP: #1714168

I'm getting a very similar error when trying to build the source out of a branch: http://pastebin.ubuntu.com/25444631/

09/01/2017 09:48:55 - ERROR:stderr: gbp:error: Pristine-tar couldn't checkout "samba_4.6.7+dfsg.orig.tar.gz": fatal: ambiguous argument '58fe526e6611a874f408d8ccc4fa22a0f6b754d4^{tree}': unknown revision or path not in the working tree.

But there are new errors after that:
09/01/2017 09:49:26 - ERROR:stderr: dpkg-buildpackage: error: fakeroot not found, either install the fakeroot

and

(...)
  File "/snap/git-ubuntu/209/gitubuntu/build.py", line 560, in fetch_orig_and_build
    if must_build:
NameError: name 'must_build' is not defined