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>

fast-import.c

@@ -931,7 +931,7 @@ static void unkeep_all_packs(void)
 		struct packed_git *p = all_packs[k];
 		snprintf(name, sizeof(name), "%s/pack/pack-%s.keep",
 			 get_object_directory(), sha1_to_hex(p->sha1));
-		unlink(name);
+		unlink_or_warn(name);
 	}
 }
 
@@ -981,7 +981,7 @@ static void end_packfile(void)
 	}
 	else {
 		close(old_p->pack_fd);
-		unlink(old_p->pack_name);
+		unlink_or_warn(old_p->pack_name);
 	}
 	free(old_p);