replace: die early if replace ref already exists

If a replace ref already exists for an object, it is
much better for the user if we error out before we
let the user edit the object, rather than after.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/replace.c

@@ -268,7 +268,8 @@ static int edit_and_replace(const char *object_ref, int force)
 {
 	char *tmpfile = git_pathdup("REPLACE_EDITOBJ");
 	enum object_type type;
-	unsigned char old[20], new[20];
+	unsigned char old[20], new[20], prev[20];
+	char ref[PATH_MAX];
 
 	if (get_sha1(object_ref, old) < 0)
 		die("Not a valid object name: '%s'", object_ref);
@@ -277,6 +278,8 @@ static int edit_and_replace(const char *object_ref, int force)
 	if (type < 0)
 		die("unable to get object type for %s", sha1_to_hex(old));
 
+	check_ref_valid(old, prev, ref, sizeof(ref), force);
+
 	export_object(old, tmpfile);
 	if (launch_editor(tmpfile, NULL, NULL) < 0)
 		die("editing object file failed");