phabricator: do not read a same revision twice

It's possible to set up non-linear dependencies in Phabricator like:

  o   D4
  |\
  | o D3
  | |
  o | D2
  |/
  o   D1

The old `phabread` code will print D1 twice. This patch adds de-duplication
to prevent that.

Test Plan:
Construct the above dependencies in a Phabricator test instance and make
sure the old code prints D1 twice while the new code won't.

contrib/phabricator.py

@@ -381,11 +381,15 b' def querydrev(repo, params, stack=False)'
             raise error.Abort(_('cannot get Differential Revision %r') % params)
         return prefetched[key]
 
+    visited = set()
     result = []
     queue = [params]
     while queue:
         params = queue.pop()
         drev = fetch(params)
+        if drev[r'id'] in visited:
+            continue
+        visited.add(drev[r'id'])
         result.append(drev)
         if stack:
             auxiliary = drev.get(r'auxiliary', {})