[PATCH 0 of 5 VERSION2] git subrepos

[Eric Eisner]

Next version of git subrepos, split into more patches.

Added some new test cases for checkout off-branch; subprocess commands fail based off return codes and print out stderr. Removing the subrepo keeps now keeps the local repository.

When the user updates to a revision off of a git branch, they are given a warning to checkout a branch before committing. There is no further warning upon unbranched commits.

Martin, I couldn't get subprocess.Popen to correctly quote the arguments, and this approach is copied directly from svnsubrepo.