cherry-pick/revert: transparently refresh index

A stat-dirty index is not a detail that ought to concern the operator
of porcelain such as "git cherry-pick".

Without this change, a cherry-pick after copying a worktree with rsync
errors out with a misleading message.

	$ git cherry-pick build/top
	error: Your local changes to 'file.h' would be overwritten by merge.  Aborting.
	Please, commit your changes or stash them before you can merge.

Noticed-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jrn

builtin/revert.c

@@ -547,6 +547,21 @@ static void prepare_revs(struct rev_info *revs)
 		die("empty commit set passed");
 }
 
+static void read_and_refresh_cache(const char *me)
+{
+	static struct lock_file index_lock;
+	int index_fd = hold_locked_index(&index_lock, 0);
+	if (read_index_preload(&the_index, NULL) < 0)
+		die("git %s: failed to read the index", me);
+	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, NULL, NULL, NULL);
+	if (the_index.cache_changed) {
+		if (write_index(&the_index, index_fd) ||
+		    commit_locked_index(&index_lock))
+			die("git %s: failed to refresh the index", me);
+	}
+	rollback_lock_file(&index_lock);
+}
+
 static int revert_or_cherry_pick(int argc, const char **argv)
 {
 	struct rev_info revs;
@@ -567,8 +582,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 			die("cherry-pick --ff cannot be used with --edit");
 	}
 
-	if (read_cache() < 0)
-		die("git %s: failed to read the index", me);
+	read_and_refresh_cache(me);
 
 	prepare_revs(&revs);
 

t/t3501-revert-cherry-pick.sh

@@ -81,6 +81,16 @@ test_expect_success 'revert after renaming branch' '
 
 '
 
+test_expect_success 'cherry-pick on stat-dirty working tree' '
+	git clone . copy &&
+	(
+		cd copy &&
+		git checkout initial &&
+		test-chmtime +40 oops &&
+		git cherry-pick added
+	)
+'
+
 test_expect_success 'revert forbidden on dirty working tree' '
 
 	echo content >extra_file &&