discovery: don't compute allfuturecommon when it won't be used

In repos with many changesets, the computation of allfuturecommon
can take a significant amount of time.  Since it's only used if
there's an obsstore, don't compute it otherwise.

mercurial/discovery.py

@@ -272,9 +272,13 b' def checkheads(repo, remote, outgoing, r'
     # If there are more heads after the push than before, a suitable
     # error message, depending on unsynced status, is displayed.
     error = None
-    allmissing = set(outgoing.missing)
-    allfuturecommon = set(c.node() for c in repo.set('%ld', outgoing.common))
-    allfuturecommon.update(allmissing)
+    # If there is no obsstore, allfuturecommon won't be used, so no
+    # need to compute it.
+    if repo.obsstore:
+        allmissing = set(outgoing.missing)
+        cctx = repo.set('%ld', outgoing.common)
+        allfuturecommon = set(c.node() for c in cctx)
+        allfuturecommon.update(allmissing)
     for branch, heads in sorted(headssum.iteritems()):
         remoteheads, newheads, unsyncedheads = heads
         candidate_newhs = set(newheads)