git-svn: pass an unambiguous ref to rev-list when grafting-branches

Some users apparently create local heads with the same basename
as the remote branch they're tracking.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-svn.perl

@@ -1086,7 +1086,7 @@ sub graft_merge_msg {
 	my ($grafts, $l_map, $u, $p, @re) = @_;
 
 	my $x = $l_map->{$u}->{$p};
-	my $rl = rev_list_raw($x);
+	my $rl = rev_list_raw("refs/remotes/$x");
 	while (my $c = next_rev_list_entry($rl)) {
 		foreach my $re (@re) {
 			my (@br) = ($c->{m} =~ /$re/g);