discovery: properly exclude locally known but filtered heads

The conditional was a bit too narrow and produced buggy result when a node was
present in both common and heads (because it pleased the discovery) and it was
locally known but filtered.

This resulted in buggy getbundle request and server side crash.

mercurial/exchange.py

@@ -946,8 +946,9 b' def _pulldiscoverychangegroup(pullop):'
         scommon = set(common)
         filteredrheads = []
         for n in rheads:
-            if n in nm and n not in scommon:
-                common.append(n)
+            if n in nm:
+                if n not in scommon:
+                    common.append(n)
             else:
                 filteredrheads.append(n)
         if not filteredrheads:

tests/test-treediscovery.t

@@ -520,7 +520,7 b' Both have new stuff in existing named br'
   "GET /?cmd=heads HTTP/1.1" 200 -
   "GET /?cmd=branches HTTP/1.1" 200 - x-hgarg-1:nodes=d8f638ac69e9ae8dea4f09f11d696546a912d961
   "GET /?cmd=between HTTP/1.1" 200 - x-hgarg-1:pairs=d8f638ac69e9ae8dea4f09f11d696546a912d961-d57206cc072a18317c1e381fb60aa31bd3401785
-  "GET /?cmd=changegroupsubset HTTP/1.1" 200 - x-hgarg-1:bases=d8f638ac69e9ae8dea4f09f11d696546a912d961&heads=d8f638ac69e9ae8dea4f09f11d696546a912d961+2c8d5d5ec612be65cdfdeac78b7662ab1696324a
+  "GET /?cmd=changegroupsubset HTTP/1.1" 200 - x-hgarg-1:bases=d8f638ac69e9ae8dea4f09f11d696546a912d961&heads=d8f638ac69e9ae8dea4f09f11d696546a912d961
   "GET /?cmd=listkeys HTTP/1.1" 200 - x-hgarg-1:namespace=phases
   "GET /?cmd=capabilities HTTP/1.1" 200 -
   "GET /?cmd=heads HTTP/1.1" 200 -