delta: skip "empty delta" optimisation for non-general case (issue6006)

Non-general delta repository cannot delta against anything than prev. So even if
the delta to prev is empty we should use it.

This is similar to the change made in bafa1c4bb7a8.

Differential Revision: https://phab.mercurial-scm.org/D5201

mercurial/revlogutils/deltas.py

@@ -593,7 +593,10 b' def _candidategroups(revlog, textlen, p1'
         group = []
         for rev in temptative:
             # skip over empty delta (no need to include them in a chain)
-            while not (rev == nullrev or rev in tested or deltalength(rev)):
+            while (revlog._generaldelta
+                   and not (rev == nullrev
+                            or rev in tested
+                            or deltalength(rev))):
                 tested.add(rev)
                 rev = deltaparent(rev)
             # filter out revision we tested already