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
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..