commit: allow editing notes in commit message editor

One workflow for git-notes is to informally keep a list of
changes from one version of a patch to another as the patch
is modified via "commit --amend" and "git rebase". Often the
most convenient time for this is while editing the commit
message, since you see it during amend, during "rebase -i"
edit stops, and when "rebase" finds a conflict.

This patch adds a "--notes" option which displays existing
notes in the commit editor (in the case of --amend), and
extracts new notes from the editor message to add to the
newly created commit.

The feature is activated only for interactive edits, so "-F"
and "-m" messages are safe from munging.