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.

Ran test suite.
Checked that the hashes of the diff content match the ones used in the new

  assertion checks.