buildbot/buildbot — pull request #267 by cdavis5x (2011-10-09 04:07:39+00:00)
Title: buildslave/git: Don't fail the whole build if git branch -M can't rename 

Previous versions of Git allowed the current branch to be renamed. For some reason, Git 1.7.7 does not allow this. All attempts to rename the current branch fail with:
```fatal: Cannot force update the current branch.```
So, if the rename fails, don't abort.

As part of this, add support for commands that can fail without causing the entire build to stop.

Fixes Trac #2127