Fix git parent diff support in diff_between_revisions

The calling contract stipulates that the parent diff should be in a
dictionary entry called 'parent_diff', not 'parent_diff_lines'.

I also snuck in a change to the git unit tests: if they don't create a
dummy .gitconfig in the new HOME, "git commit" dies later, triggering all
sorts of unhappiness in nose.

Testing done: Ran post-review with --revision-range.
Fixes bug 3125
Reviewed at https://reviews.reviewboard.org/r/4870/