_get-commit-branch more accurately determines branch

The previous method was inaccurate and non-deterministic; see the comment in the
docblock for additional details on the new implementation, which will be
improvde upon in the future.
Re-merge will now contain short hash and branch name
git-remerge now returns to cwd after init

This prevents get from becoming confused when performing merge operations on a
non-bare repo, potentially while in another repo.