merge-recursive:make_room_for_directories - work around dumb compilers

Some vintage of gcc does not seem to notice last_len is only used when
last_file is already set to non-NULL at which point last_len is also
set.

Noticed on FreeBSD 8

Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -365,7 +365,7 @@ static void make_room_for_directories_of_df_conflicts(struct merge_options *o,
 	 * below the corresponding directory.
 	 */
 	const char *last_file = NULL;
-	int last_len;
+	int last_len = 0;
 	struct stage_data *last_e;
 	int i;