Make sure lockfiles are unlocked when dying on SIGPIPE

We cleaned up lockfiles upon receiving the usual suspects HUP, TERM, QUIT
but a wicked user could kill us of asphyxiation by piping our output to a
pipe that does not read.  Protect ourselves by catching SIGPIPE and clean
up the lockfiles as well in such a case.

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

lockfile.c

@@ -140,6 +140,7 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 			signal(SIGHUP, remove_lock_file_on_signal);
 			signal(SIGTERM, remove_lock_file_on_signal);
 			signal(SIGQUIT, remove_lock_file_on_signal);
+			signal(SIGPIPE, remove_lock_file_on_signal);
 			atexit(remove_lock_file);
 		}
 		lk->owner = getpid();