git-instaweb: Remove pidfile after stopping web server

This way running e.g. "git instaweb" after "git instaweb --stop" would not try to kill already stopped web server. This is probably important only for those web servers that are "daemonized" by git-instaweb itself, i.e. for those where it is git-instaweb that creates pidfile. Currently it is includes only 'mongoose' web server, but it would also include 'plackup' web server (added in later commit). Signed-off-by: Jakub Narebski <jnareb@gmail.com> Acked-by: Petr Baudis <pasky@suse.cz> Acked-by: Eric Wong <normalperson@yhbt.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>