changegroup: invert conditional and dedent

I don't like else-less blocks that are indented for no reason.

Differential Revision: https://phab.mercurial-scm.org/D4217

mercurial/changegroup.py

@@ -1281,7 +1281,9 b' class cgpacker(object):'
             filenodes = [n for n in linkrevnodes
                          if flr(frev(n)) not in commonrevs]
 
-            if filenodes:
+            if not filenodes:
+                continue
+
                 if self._ellipses:
                     revs = _sortnodesellipsis(filerevlog, filenodes,
                                               cl, lookupfilelog)