As part of the FreeBSD integration of Phabricator + Jenkins,
we ran an experiment where instead of checking out
the source code for our tree into the root ${WORKSPACE} directory,
we checked out the source code to some arbitrary ${WORKSPACE}/subdirectory/ .

We found that the phabricator-plugin calls "arc patch", but it assumes the
source code is in the root ${WORKSPACE} directory. It would be nice if
we could specify to the phabricator-plugin a subdirectory instead of assuming the code is in
the root directory.