"git-merge": allow fast-forwarding in a stat-dirty tree

We used to refresh the index to clear stat-dirtyness before a fast-forward
merge.  Recent C rewrite forgot to do this.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -566,6 +566,7 @@ static int checkout_fast_forward(unsigned char *head, unsigned char *remote)
 
 	if (read_cache_unmerged())
 		die("you need to resolve your current index first");
+	refresh_cache(REFRESH_QUIET);
 
 	fd = hold_locked_index(lock_file, 1);
 
@@ -936,7 +937,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			hex,
 			find_unique_abbrev(remoteheads->item->object.sha1,
 			DEFAULT_ABBREV));
-		refresh_cache(REFRESH_QUIET);
 		strbuf_init(&msg, 0);
 		strbuf_addstr(&msg, "Fast forward");
 		if (have_message)

t/t7600-merge.sh

@@ -488,4 +488,14 @@ test_expect_success 'merge c1 with c1 and c2' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge fast-forward in a dirty tree' '
+       git reset --hard c0 &&
+       mv file file1 &&
+       cat file1 >file &&
+       rm -f file1 &&
+       git merge c2
+'
+
+test_debug 'gitk --all'
+
 test_done