Improve patch to use git --exec-path: add to PATH instead.

Improve patch to use git --exec-path: add to PATH instead.

If you (like me) are using a modified git straight out of its source
directory (ie. without installing), then --exec-path isn't actually correct.
Add it to the PATH instead, so if it is correct, it'll work, but if it's
not, we fall back to the previous behaviour.