git-revert: record the parent against which a revert was made

As described in Documentation/howto/revert-a-faulty-merge.txt, re-merging
from a previously reverted a merge of a side branch may need a revert of
the revert beforehand.  Record against which parent the revert was made in
the commit, so that later the user can figure out what went on.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-revert.c

@@ -352,6 +352,11 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 		add_to_msg(oneline_body + 1);
 		add_to_msg("\"\n\nThis reverts commit ");
 		add_to_msg(sha1_to_hex(commit->object.sha1));
+
+		if (commit->parents->next) {
+			add_to_msg(", reversing\nchanges made to ");
+			add_to_msg(sha1_to_hex(parent->object.sha1));
+		}
 		add_to_msg(".\n");
 	} else {
 		base = parent;