tests/test-empty-dir.t

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

  $ hg init

  $ echo 123 > a
  $ hg add a
  $ hg commit -m "first" a

  $ mkdir sub
  $ echo 321 > sub/b
  $ hg add sub/b
  $ hg commit -m "second" sub/b

  $ cat sub/b
  321

  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ cat sub/b 2>/dev/null || echo "sub/b not present"
  sub/b not present

  $ test -d sub || echo "sub not present"
  sub not present