revert/cherry-pick: work on merge commits as well

Usually you cannot revert a merge because you do not know which
side of the merge should be considered the mainline (iow, what
change to reverse).

With this patch, cherry-pick and revert learn -m (--mainline)
option that lets you specify the parent number (starting from 1)
of the mainline, so that you can:

	git revert -m 1 $merge

to reverse the changes introduced by the $merge commit relative
to its first parent, and:

	git cherry-pick -m 2 $merge

to replay the changes introduced by the $merge commit relative
to its second parent.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 35 insertions, 7 deletions

diff --git a/builtin-revert.c b/builtin-revert.c
index a655c8ee2a..bfed69d7e2 100644
--- a/builtin-revert.c
+++ b/builtin-revert.c
@@ -19,9 +19,9 @@
  * Copyright (c) 2005 Junio C Hamano
  */
 
-static const char *revert_usage = "git-revert [--edit | --no-edit] [-n] <commit-ish>";
+static const char *revert_usage = "git-revert [--edit | --no-edit] [-n] [-m parent-number] <commit-ish>";
 
-static const char *cherry_pick_usage = "git-cherry-pick [--edit] [-n] [-r] [-x] <commit-ish>";
+static const char *cherry_pick_usage = "git-cherry-pick [--edit] [-n] [-m parent-number] [-r] [-x] <commit-ish>";
 
 static int edit;
 static int replay;
@@ -29,6 +29,7 @@ static enum { REVERT, CHERRY_PICK } action;
 static int no_commit;
 static struct commit *commit;
 static int needed_deref;
+static int mainline;
 
 static const char *me;
 
@@ -58,6 +59,12 @@ static void parse_options(int argc, const char **argv)
 		else if (!strcmp(arg, "-x") || !strcmp(arg, "--i-really-want-"
 				"to-expose-my-private-commit-object-name"))
 			replay = 0;
+		else if (!strcmp(arg, "-m") || !strcmp(arg, "--mainline")) {
+			if (++i >= argc ||
+			    strtol_i(argv[i], 10, &mainline) ||
+			    mainline <= 0)
+				usage(usage_str);
+		}
 		else if (strcmp(arg, "-r"))
 			usage(usage_str);
 	}
@@ -234,7 +241,7 @@ static int merge_recursive(const char *base_sha1,
 static int revert_or_cherry_pick(int argc, const char **argv)
 {
 	unsigned char head[20];
-	struct commit *base, *next;
+	struct commit *base, *next, *parent;
 	int i;
 	char *oneline, *reencoded_message = NULL;
 	const char *message, *encoding;
@@ -269,8 +276,29 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 
 	if (!commit->parents)
 		die ("Cannot %s a root commit", me);
-	if (commit->parents->next)
-		die ("Cannot %s a multi-parent commit.", me);
+	if (commit->parents->next) {
+		/* Reverting or cherry-picking a merge commit */
+		int cnt;
+		struct commit_list *p;
+
+		if (!mainline)
+			die("Commit %s is a merge but no -m option was given.",
+			    sha1_to_hex(commit->object.sha1));
+
+		for (cnt = 1, p = commit->parents;
+		     cnt != mainline && p;
+		     cnt++)
+			p = p->next;
+		if (cnt != mainline || !p)
+			die("Commit %s does not have parent %d",
+			    sha1_to_hex(commit->object.sha1), mainline);
+		parent = p->item;
+	} else if (0 < mainline)
+		die("Mainline was specified but commit %s is not a merge.",
+		    sha1_to_hex(commit->object.sha1));
+	else
+		parent = commit->parents->item;
+
 	if (!(message = commit->buffer))
 		die ("Cannot get commit message for %s",
 				sha1_to_hex(commit->object.sha1));
@@ -299,14 +327,14 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 		char *oneline_body = strchr(oneline, ' ');
 
 		base = commit;
-		next = commit->parents->item;
+		next = parent;
 		add_to_msg("Revert \"");
 		add_to_msg(oneline_body + 1);
 		add_to_msg("\"\n\nThis reverts commit ");
 		add_to_msg(sha1_to_hex(commit->object.sha1));
 		add_to_msg(".\n");
 	} else {
-		base = commit->parents->item;
+		base = parent;
 		next = commit;
 		set_author_ident_env(message);
 		add_message_to_msg(message);