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>

Documentation/git-fast-import.txt

@@ -898,6 +898,12 @@ import-marks::
 	second, an --import-marks= command-line option overrides
 	any "feature import-marks" command in the stream.
 
+notes::
+	Require that the backend support the 'notemodify' (N)
+	subcommand to the 'commit' command.
+	Versions of fast-import not supporting notes will exit
+	with a message indicating so.
+
 `option`
 ~~~~~~~~
 Processes the specified option so that git fast-import behaves in a

fast-import.c

@@ -2821,6 +2821,8 @@ static int parse_one_feature(const char *feature, int from_stream)
 		relative_marks_paths = 0;
 	} else if (!prefixcmp(feature, "force")) {
 		force_update = 1;
+	} else if (!strcmp(feature, "notes")) {
+		; /* do nothing; we have the feature */
 	} else {
 		return 0;
 	}

t/t9301-fast-import-notes.sh

@@ -120,6 +120,7 @@ test_expect_success 'add notes with simple M command' '
 
 test_tick
 cat >input <<INPUT_END
+feature notes
 commit refs/notes/test
 committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
 data <<COMMIT