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

hgext/phabricator.py

@@ -515,6 +515,7 b' def userphids(repo, names):'
          [(b'r', b'rev', [], _(b'revisions to send'), _(b'REV')),
           (b'', b'amend', True, _(b'update commit messages')),
           (b'', b'reviewer', [], _(b'specify reviewers')),
+          (b'', b'blocker', [], _(b'specify blocking reviewers')),
           (b'm', b'comment', b'',
            _(b'add a comment to Revisions with new/updated Diffs')),
           (b'', b'confirm', None, _(b'ask for confirmation before sending'))],
@@ -568,8 +569,15 b' def phabsend(ui, repo, *revs, **opts):'
 
     actions = []
     reviewers = opts.get(b'reviewer', [])
+    blockers = opts.get(b'blocker', [])
+    phids = []
     if reviewers:
-        phids = userphids(repo, reviewers)
+        phids.extend(userphids(repo, reviewers))
+    if blockers:
+        phids.extend(map(
+            lambda phid: b'blocking(%s)' % phid, userphids(repo, blockers)
+        ))
+    if phids:
         actions.append({b'type': b'reviewers.add', b'value': phids})
 
     drevids = [] # [int]