patch.py: re-add the ability to use an external patch program This is now invoked by default only if ui.patch is set. Otherwise, we use our built-in patch. If that fails because it can't find any valid hunks, we'll fall back to trying the external patch command.

#header#
# HG changeset patch
# User #author#
# Date #date|hgdate#
# Node ID #node#
#parent%changesetparent#
#desc#

#diff#