commit --amend and amend command in core and extensions rely on
cmdutil.amend() for amending a commit. So the logic to add a note to amend must
reside here. This patch assumes that note will be passed in opts dictionary to
the function and it will be passed to cleanupnodes and then createmarkers to
store the note in the obsmarker metadata.
After this patch, note can be stored on an amend changeset by passing notes as a
part of opts to cmdutil.amend().