Introduce commit notes

Introduce commit notes

Commit notes are blobs which are shown together with the commit
message.  These blobs are taken from the notes ref, which you can
configure by the config variable core.notesRef, which in turn can
be overridden by the environment variable GIT_NOTES_REF.

The notes ref is a branch which contains "files" whose names are
the names of the corresponding commits (i.e. the SHA-1).

The rationale for putting this information into a ref is this: we
want to be able to fetch and possibly union-merge the notes,
maybe even look at the date when a note was introduced, and we
want to store them efficiently together with the other objects.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>