revert: allow single-pick in the middle of cherry-pick sequence

After messing up a difficult conflict resolution in the middle of a
cherry-pick sequence, it can be useful to be able to

	git checkout HEAD . && git cherry-pick that-one-commit

to restart the conflict resolution. The current code however errors out
saying that another cherry-pick is already in progress.

Suggested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -1072,6 +1072,12 @@ static int sequencer_continue(struct replay_opts *opts)
 	return pick_commits(todo_list, opts);
 }
 
+static int single_pick(struct commit *cmit, struct replay_opts *opts)
+{
+	setenv(GIT_REFLOG_ACTION, action_name(opts), 0);
+	return do_pick_commit(cmit, opts);
+}
+
 static int pick_revisions(struct replay_opts *opts)
 {
 	struct commit_list *todo_list = NULL;
@@ -1096,6 +1102,26 @@ static int pick_revisions(struct replay_opts *opts)
 	if (opts->subcommand == REPLAY_CONTINUE)
 		return sequencer_continue(opts);
 
+	/*
+	 * If we were called as "git cherry-pick <commit>", just
+	 * cherry-pick/revert it, set CHERRY_PICK_HEAD /
+	 * REVERT_HEAD, and don't touch the sequencer state.
+	 * This means it is possible to cherry-pick in the middle
+	 * of a cherry-pick sequence.
+	 */
+	if (opts->revs->cmdline.nr == 1 &&
+	    opts->revs->cmdline.rev->whence == REV_CMD_REV &&
+	    opts->revs->no_walk &&
+	    !opts->revs->cmdline.rev->flags) {
+		struct commit *cmit;
+		if (prepare_revision_walk(opts->revs))
+			die(_("revision walk setup failed"));
+		cmit = get_revision(opts->revs);
+		if (!cmit || get_revision(opts->revs))
+			die("BUG: expected exactly one commit from walk");
+		return single_pick(cmit, opts);
+	}
+
 	/*
 	 * Start a new cherry-pick/ revert sequence; but
 	 * first, make sure that an existing one isn't in

t/t3510-cherry-pick-sequence.sh

@@ -50,6 +50,18 @@ test_expect_success 'cherry-pick persists data on failure' '
 	test_path_is_file .git/sequencer/opts
 '
 
+test_expect_success 'cherry-pick mid-cherry-pick-sequence' '
+	pristine_detach initial &&
+	test_must_fail git cherry-pick base..anotherpick &&
+	test_cmp_rev picked CHERRY_PICK_HEAD &&
+	# "oops, I forgot that these patches rely on the change from base"
+	git checkout HEAD foo &&
+	git cherry-pick base &&
+	git cherry-pick picked &&
+	git cherry-pick --continue &&
+	git diff --exit-code anotherpick
+'
+
 test_expect_success 'cherry-pick persists opts correctly' '
 	pristine_detach initial &&
 	test_must_fail git cherry-pick -s -m 1 --strategy=recursive -X patience -X ours base..anotherpick &&