Tests fail when Git is missing from the system or when run not from a Git directory

[danielrozenberg]

This means that I can't run the tests in an automated, hermetic system when I include the code in our third_party repository :)

Log for make tdd when Git executable is missing: https://gist.github.com/danielrozenberg/abd898f14bd42f2bcc1a60d0b2a85ab0

Log for make tdd when executing tests not from a Git directory: https://gist.github.com/danielrozenberg/b3487a6c895b90b6ce49a4ec69ffe757

Tangentially related, I'm curious why you chose to use subprocess.Popen('git', …) as opposed to including GitPython or another Git lib for python