This commit adds the utility testing function TempDir, which provides a
tester with a temporary directory as well as a means of cleaning up that
directory. The new function is implemented everywhere that makes sense
throughout the code base.
This also cleans up the directories left behind by go-git's testing
fixtures.
Some light refactoring was also performed in this change.
Change-Id: I754484934660487140f57671bacb5463cf669e3e
091fa09a23