Deserialize commit object file contents

- Complete the get_commit_object() function in LocalRepoApi.
  - Parse author, committer, date authored, and date committed
  - Do not parse tree right now, as we have no use for it in the
    foreseeable future (wow...I think that's the first time I've ever
    seen the word foreseeable written out and it's pretty weird, don't
    ya think?)