tests: split out ~1/3 of test-graft.t test-graft.t is ~2400 lines and takes 34s to run. This patch moves the last ~1/3 of it to a separate file. The parts now run in 22s + 13s. On top of that, we can remove the #testcases from the old file, so it's only 22s + 2*13s instead of the 2*34s it was before. Differential Revision: https://phab.mercurial-scm.org/D7802

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