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 add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status