Optimization

Waiting to bump to libgit2/libgit2#1630 after it's been merged.

This optimizes various things, and fixes a memory leak caused by the naming of `GTDiffDelta.newFile`.

**To do**:
 - [x] Use `git_reference_shorthand` or `git_branch_name` within `-[GTBranch remoteName]`.
 - [x] Unit test new `GTBranch` and `GTReference` methods.