fast-import: introduce "feature notes" command

fast-import: introduce "feature notes" command

Here is a 'feature' command for streams to use to require support for
the notemodify (N) command.

When the 'feature' facility was introduced (v1.7.0-rc0~95^2~4,
2009-12-04), the notes import feature was old news (v1.6.6-rc0~21^2~8,
2009-10-09) and it was not obvious it deserved to be a named feature.
But now that is clear, since all major non-git fast-import backends
lack support for it.

Details: on git version with this patch applied, any "feature notes"
command in the features/options section at the beginning of a stream
will be treated as a no-op.  On fast-import implementations without
the feature (and older git versions), the command instead errors out
with a message like

	This version of fast-import does not support feature notes.

So by declaring use of notes at the beginning of a stream, frontends
can avoid wasting time and other resources when the backend does not
support notes.  (This would be especially important for backends that
do not support rewinding history after a botched import.)

Improved-by: Thomas Rast <trast@student.ethz.ch>
Improved-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>