Allow writing a NoteMap back to the repository

This is necessary to allow applications to wrap the note tree in
a commit and update the note branch with the new state.

Change-Id: Idbd7ead4a1b16ae2b64a30a4a01a29cfed548cdf
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
7 files changed