[RFC] fast-import: notemodify (N) command

[Jonathan Nieder <jrnieder@gmail.com>]

Dear fast importers,

Another week, another fast-import protocol extension.

Most DVCSes do not allow one to non-disruptively change the log
message for a commit.  But sometimes people want to attach information to a
commit after the fact:

 - whether it was tested and worked correctly
 - who liked or disliked the commit (Acked-by, Reviewed-by)
 - corresponding revision number after export to another version
   control system
 - bug number
 - corresponding compiled binary

The N command allows such notes to be attached to commits, like so:

 1. first the commit is imported as usual (let's say it's ":1").
 2. commit annotations are added separately, like so:

	commit refs/notes/commits
	committer A. U. Thor <author@example.com> Mon, 31 Jan 2011 12:15:59 -0600
	data <<END
	Notes after review.
	END

	N inline :1
	data <<END
	Acked-by: me
	END

Details:

 - there can be multiple categories of notes: "refs/notes/commits"
   contains ordinary addenda to the commit message, but one might also
   see refs/notes/bugzilla, refs/notes/svn-commit, and so on.

 - each commit gets at most one blob of notes in each category.  Later
   notemodify (N) commands overwrite the effect from earlier ones.

 - the syntax of a notemodify command is as follows:

	'N' sp <dataref> sp <committish> lf

   The <dataref> represents a blob with the annotations to be used
   ("inline" is allowed, too, just like with filemodify).  The
   <committish> can be any expression allowed in a 'from' command
   (branch name, mark reference :<idnum>, other commit name) and
   represents the commit that is to be annotated.

 - this has been supported in git since v1.6.6.  There is no
   "feature" for it --- I don't think the feature declaration
   facility existed yet.

Do other DVCSes support something like this?  Should it get a
feature name?

Jonathan