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
mkdir src
echo a > src/a.c
hg ci -Ama -d "10000000 0"
hg mv src source
hg ci -Ammove -d "1000000 0"
hg co -C 0
echo new > src/a.c
echo compiled > src/a.o
hg ci -mupdate -d "1000000 0"
hg st
hg merge
hg st