sequencer: beautify subject of reverts of reverts

Instead of generating a silly-looking `Revert "Revert "foo""`, make it
a more humane `Reapply "foo"`.

This is done for two reasons:
- To cover the actually common case of just a double revert.
- To encourage people to rewrite summaries of recursive reverts by
  setting an example (a subsequent commit will also do this explicitly
  in the documentation).

To achieve these goals, the mechanism does not need to be particularly
sophisticated. Therefore, more complicated alternatives which would
"compress more efficiently" have not been implemented.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2249,6 +2249,8 @@ static int do_pick_commit(struct repository *r,
 	 */
 
 	if (command == TODO_REVERT) {
+		const char *orig_subject;
+
 		base = commit;
 		base_label = msg.label;
 		next = parent;
@@ -2256,6 +2258,15 @@ static int do_pick_commit(struct repository *r,
 		if (opts->commit_use_reference) {
 			strbuf_addstr(&msgbuf,
 				"# *** SAY WHY WE ARE REVERTING ON THE TITLE LINE ***");
+		} else if (skip_prefix(msg.subject, "Revert \"", &orig_subject) &&
+			   /*
+			    * We don't touch pre-existing repeated reverts, because
+			    * theoretically these can be nested arbitrarily deeply,
+			    * thus requiring excessive complexity to deal with.
+			    */
+			   !starts_with(orig_subject, "Revert \"")) {
+			strbuf_addstr(&msgbuf, "Reapply \"");
+			strbuf_addstr(&msgbuf, orig_subject);
 		} else {
 			strbuf_addstr(&msgbuf, "Revert \"");
 			strbuf_addstr(&msgbuf, msg.subject);