revset: return early when revs is empty

By introducing an early return in _revancestors() when revs is empty,
we make it so inputrev is never None, which simplifies the subsequent
code.

mercurial/revset.py

@@ -25,12 +25,12 def _revancestors(repo, revs, followfirs
     cl = repo.changelog
 
     def iterate():
-        revqueue, inputrev = None, None
-        h = []
-
         revs.sort(reverse=True)
         revqueue = util.deque(revs)
-        if revqueue:
+        if not revqueue:
+            return
+
+        h = []
             inputrev = revqueue.popleft()
             heapq.heappush(h, -inputrev)
 
@@ -38,7 +38,7 def _revancestors(repo, revs, followfirs
         while h:
             current = -heapq.heappop(h)
             if current not in seen:
-                if inputrev and current == inputrev:
+                if current == inputrev:
                     if revqueue:
                         inputrev = revqueue.popleft()
                         heapq.heappush(h, -inputrev)