overlwayworkingctx: remove doubly bad reference to wrapped ctx for copies
`_wrappedctx` lives on overlwayworkingctx, not on the repo object, so
we should access it as `._wrappedctx`, not `._repo._wrappedctx`. More
importantly, the overlayworkingctx is relative to its base, not
including it, so the copies returned should not include copies made in
the base.
Differential Revision: https://phab.mercurial-scm.org/D7801
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown