Start improving rbtools test suite.

There are a bunch of things that make working with the rbtools test suite
annoying. This cleans up some of them:
* Change some docstrings to always use "Testing ...", so we get consistent
  output.
* Reformat some docstrings so we don't truncate test case names.
* Create a bare git repo in a testdata directory so we don't have to initialize
  a new repo every time we do `setUp()` (note that we still clone it for each
  test). This will also make it much easier to add new branches and commits to
  test other features.
* Create a static home directory to use for things like .gitconfig, so we don't
  have to create them each time.
* Get rid of all the inline diff content and just check if the md5 hash matches
  what we expect.

Testing done:
* Ran test suite.
* Checked that the hashes of the diff content match the ones used in the new
  assertion checks.

Reviewed at https://reviews.reviewboard.org/r/5111/