lib/codereview: fix with more recent hg revisions.

I've Mercurial version 3.2 and hg submit fails with:

  File "/home/agl/devel/go/lib/codereview/codereview.py", line 3567, in get_hg_status
    ret = hg_commands.status(fui, self.repo, *[], **{'rev': [rev], 'copies': True})
  File "/usr/lib/python2.7/site-packages/mercurial/commands.py", line 5714, in status
    fm = ui.formatter('status', opts)
  File "/home/agl/devel/go/lib/codereview/codereview.py", line 3464, in formatter
    return plainformatter(self, topic, opts)
  File "/usr/lib/python2.7/site-packages/mercurial/formatter.py", line 57, in __init__
    if ui.debugflag:
  AttributeError: 'FakeMercurialUI' object has no attribute 'debugflag'

This change dumbly adds a boolean debugflag and that seems to work.

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/167410043