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
#require grey
(this should use the actual black as soon as possible)
$ cd $RUNTESTDIR/..
$ python3 contrib/grey.py --config=black.toml --check --diff `hg files 'set:**.py - hgext/fsmonitor/pywatchman/** - mercurial/thirdparty/** - "contrib/python-zstandard/**" - contrib/grey.py'`