Rewrite tests for GTBlob.
Use `git_blob_create_fromdisk` instead of going through the `fromworkdir` version.

Because I don't want to handle the pain of relativizing the passed URL.
Asserts.
Move the contents of `GTBranchTest` where they belong.
Remove `GTBranchTest`.

I know there's still a test in here, but I'm not sure what it's testing since we're ARC now…
Convert `GTCommitTest` to Expecta.
Convert `GTReferenceTest`.
Convert `GTRepositoryTest`.
Cleanup `GTBlob`'s header.
Make `error` a local variable.
Expect errors to be nil here.
Reindent commented test.
Rename test.
Cruft--;
Dot-syntaxify.
Rename `isValidReference` and define it at declaration time.
Make `error` a local variable.
Fix the `lookupObjectByRefspec:error:` name.
Whitespace.
Test that the returned branches are the expected ones.