Refactored ref parsing to make it more consistent and centralized.

Requires a migration of how tips are stored so that they use gitify refs (a
concept introduced in this commit) to be consistent with the rest of the code,
instead of a special format.
Report correct (instead of unknown) hashes when importing refs into git
Fixed git push messages test
Merge pull request #65 from alexsydell/refs

Prettier git messages and another big refactor of ref parsing