Apply patches from Phabricator in a smarter way

[marco-c]

We are currently doing

libmozevent/libmozevent/mercurial.py

Line 218 in ddb136d

self.repo.import_(

, we could instead take inspiration from Lando.
@cgsheeh said:

Lando does an hg import --no-commit -s 95 and if that fails, it then runs a hg import --no-commit --config ui.patch=patch -s 95, which falls back to using the patch command

Related to mozilla/code-review#2057.