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-prune.c

@@ -27,7 +27,7 @@ static int prune_tmp_object(const char *path, const char *filename)
 	}
 	printf("Removing stale temporary file %s\n", fullpath);
 	if (!show_only)
-		unlink(fullpath);
+		unlink_or_warn(fullpath);
 	return 0;
 }
 
@@ -47,7 +47,7 @@ static int prune_object(char *path, const char *filename, const unsigned char *s
 		       (type > 0) ? typename(type) : "unknown");
 	}
 	if (!show_only)
-		unlink(fullpath);
+		unlink_or_warn(fullpath);
 	return 0;
 }