buildman: Allow branch names which conflict with directories

At present if you try to use buildman with the branch 'test' it will
complain that it is unsure whether you mean the branch or the directory.
This is a feature of the 'git log' command that buildman uses. Fix it
by resolving the ambiguity.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>