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.
Test the complete reference array in one step.
Clarify comment.
Test the actual error domain & code.