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.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.