Get commit contents in LocalRepo via GitTerminal

- Use the GitTerminal to get commit object file contents in the
  LocalRepository's get_commit_object() method, instead of unzipping
  the file manually with zlib, which didn't work on packfiles. Close
  Issues #1 and #2 on GitHub.
- Change Sha1 str() method to return just the raw SHA-1, which seems
  more intuitive