replace direct calls to unlink(2) with unlink_or_warn

This helps to notice when something's going wrong, especially on
systems which lock open files.

I used the following criteria when selecting the code for replacement:
- it was already printing a warning for the unlink failures
- it is in a function which already printing something or is
  called from such a function
- it is in a static function, returning void and the function is only
  called from a builtin main function (cmd_)
- it is in a function which handles emergency exit (signal handlers)
- it is in a function which is obvously cleaning up the lockfiles

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

entry.c

@@ -35,7 +35,7 @@ static void create_directories(const char *path, int path_len,
 		 */
 		if (mkdir(buf, 0777)) {
 			if (errno == EEXIST && state->force &&
-			    !unlink(buf) && !mkdir(buf, 0777))
+			    !unlink_or_warn(buf) && !mkdir(buf, 0777))
 				continue;
 			die("cannot create directory at %s", buf);
 		}