Apply git notes to store hg sha1 on clone and pull, ref #4

* The Hg sha1 is written to refs/notes/gitifyhg, it can be viewed
  using `git log --notes=gitifyhg`.
* Only applies notes during clone and pull (not applied by push,
  even after subsequent pull).
* Sets notes parent to match last note applied by us. If there are
  multiple remotes, this would yield multiple notes heads.