cygwin: stop forcing core.filemode=false

We force core.filemode=false since c869753e (Force core.filemode to false on Cygwin., 2006-12-30), even when the repository is on a filesystem on which Cygwin can give us trustable filemodes, because many native Windows applications the users use to edit files in the working tree tend to (re)create files with executable bit randomly set or reset. However, binary distribution of Git that is supplied by the downstream project to its users has been built without this consideration. Drop NO_TRUSTABLE_FILEMODE from our default configuration so that hand-compiled Git out of box will match theirs. Signed-off-by: Mark Levedahl <mlevedahl@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>