format-patch: add --range-diff option to embed diff in cover letter

When submitting a revised version of a patch series, it can be helpful
(to reviewers) to include a summary of changes since the previous
attempt in the form of a range-diff, however, doing so involves manually
copy/pasting the diff into the cover letter.

Add a --range-diff option to automate this process. The argument to
--range-diff specifies the tip of the previous attempt against which to
generate the range-diff. For example:

    git format-patch --cover-letter --range-diff=v1 -3 v2

(At this stage, the previous attempt and the patch series being
formatted must share a common base, however, a subsequent enhancement
will make it possible to specify an explicit revision range for the
previous attempt.)

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -218,6 +218,11 @@ struct rev_info {
 	const struct object_id *idiff_oid2;
 	const char *idiff_title;
 
+	/* range-diff */
+	const char *rdiff1;
+	const char *rdiff2;
+	int creation_factor;
+
 	/* commit counts */
 	int count_left;
 	int count_right;