This updates the Git unit tests to build a GitClient in each test

where it's needed, rather than in setUp(). This is in prepration for

new tests that will need to handle client construction differently.
This also cleans up a lot of the assertions to be less verbose.
We had a lot of combinations of checking instance type, key presence,

and then values, but this is far less useful (less accurate) than just

checking dictionaries for equality. We missed things because we weren't

doing these checks.
We also had MD5-based checks for diffs, but these make diagnostics

difficult.
Given the work being done already in each function, I felt this was the

best time to clean this code up.

Git unit tests pass.