Store hg sha1 using git notes (partial implementation of #4)

This writes the hg sha1 into `git notes`, which can be queried using commands like

    git log --notes=hg

The implementation currently favors safety by writing to a private notes ref `refs/notes/hg-REMOTE` where `REMOTE` is the configured remote name (e.g., `origin`).  (I expect this to eventually change to a hash of the remote URL as discussed in issue #14.)  This implementation currently requires the user to somehow run

    git notes --ref hg merge hg-REMOTE*

as there does not appear to be a safe way for a remote helper to update the shared ref itself. I asked about this on the git mailing list (http://thread.gmane.org/gmane.comp.version-control.git/214802) and will hold off on hacks to commit the final merge.

Currently, notes are not applied in push operations, but they _are_ applied in subsequent fetches.