tests/test-extra-filelog-entry.t

engine: prevent multiple checking of re-delta-multibase

The _perform_clone function is called for each revlog cloned, hence we should
prevent this function call overhead.

Differential Revision: https://phab.mercurial-scm.org/D9669

Issue351: mq: qrefresh can create extra revlog entry

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init
  $ hg qinit

  $ echo b > b
  $ hg ci -A -m foo
  adding b

  $ echo cc > b
  $ hg qnew -f foo.diff
  $ echo b > b
  $ hg qrefresh

  $ hg debugindex b
     rev linkrev nodeid       p1           p2
       0       0 1e88685f5dde 000000000000 000000000000