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