phabricator: switch to the creatediff endpoint
This lets the extension submit binary files, as well as set branch info so that
it is exposed in the Phabricator interface.
Differential Revision: https://phab.mercurial-scm.org/D7053
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL