phabricator: add --blocker argument to phabsend to specify blocking reviewers
The way to signal to Conduit that a reviewer is considered blocking is just to
wrap their PHID in "blocking()" when including it in the list of PHIDs passed
to `reviewers.add`.
arc doesn't have a --blocker, instead one is supposed to append a '!' to the
end of reviewer names (I think reviewers are usually added in an editor rather
than the command line, where '!'s can be more hazardous).
moz-phab (Mozilla's arcanist wrapper) does have a --blocker argument, and being
explicit like this is also more discoverable. Even `arc diff`'s help doesn't
seem to mention the reviewer! syntax.
Differential Revision: https://phab.mercurial-scm.org/D6512