tests/test-issue619

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

mkdir t
cd t
hg init
echo a > a
hg ci -Ama -d '1000000000 0'
echo b > b
hg branch b
hg ci -Amb -d '1000000000 0'
hg co -C 0

echo fast-forward
hg merge b
hg ci -Ammerge -d '1000000000 0'

echo bogus fast-forward should fail
hg merge b

echo done