Allow diff and index commands to be interrupted

So far, e.g. git-update-index --refresh was basically uninterruptable
by ctrl-c, since it hooked the SIGINT handler, but that handler would
only unlink the lockfile but not actually quit. This makes it propagate
the signal to the default handler.

Note that I expected it to work without resetting the signal handler to
SIG_DFL, but without that it ended in an infinite loop of tgkill()s -
is my glibc violating SUS or what?

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

index.c

@@ -18,6 +18,8 @@ static void remove_lock_file(void)
 static void remove_lock_file_on_signal(int signo)
 {
 	remove_lock_file();
+	signal(SIGINT, SIG_DFL);
+	raise(signo);
 }
 
 int hold_index_file_for_update(struct cache_file *cf, const char *path)