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.

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true