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>

builtin-rerere.c

@@ -116,7 +116,7 @@ int cmd_rerere(int argc, const char **argv, const char *prefix)
 			if (!has_rerere_resolution(name))
 				unlink_rr_item(name);
 		}
-		unlink(git_path("rr-cache/MERGE_RR"));
+		unlink_or_warn(git_path("rr-cache/MERGE_RR"));
 	} else if (!strcmp(argv[1], "gc"))
 		garbage_collect(&merge_rr);
 	else if (!strcmp(argv[1], "status"))