revset: fix generatorset race condition

If two things were iterating over a generatorset at the same time, they could
miss out on the things the other was generating, resulting in incomplete
results. This fixes it by making it possible for two things to iterate at once,
by always checking the _genlist at the beginning of each iteration.

I was only able to repro it with pending changes from my other commits, but they
aren't ready yet. So I'm unable to add a test for now.

mercurial/revset.py

@@ -2624,10 +2624,8 b' class _generatorset(object):'
         gen: a generator producing the values for the generatorset.
         """
         self._gen = gen
-        self._iter = iter(gen)
         self._cache = {}
         self._genlist = baseset([])
-        self._iterated = False
         self._finished = False
 
     def __contains__(self, x):
@@ -2639,28 +2637,30 b' class _generatorset(object):'
             if l == x:
                 return True
 
-        self._finished = True
         self._cache[x] = False
         return False
 
     def __iter__(self):
-        if self._iterated:
-            # At least a part of the list should be cached if iteration has
-            # started over the generatorset.
-            for l in self._genlist:
-                yield l
-
-        for item in self._consumegen():
-            yield item
+        if self._finished:
+            for x in self._genlist:
+                yield x
+            return
+
+        i = 0
+        genlist = self._genlist
+        consume = self._consumegen()
+        while True:
+            if i < len(genlist):
+                yield genlist[i]
+            else:
+                yield consume.next()
+            i += 1
 
     def _consumegen(self):
-        self._iterated = True
-
         for item in self._gen:
             self._cache[item] = True
             self._genlist.append(item)
             yield item
-
         self._finished = True
 
     def set(self):