amend: don't remove unselected removals from memctx When there are removed files in the working copy and they are not selected to be amended into the parent, the `filectxfn` we create for the `memctx` would still return `None` before this patch. That's clearly incorrect; we should return the `filectx` from the unamended commit. Somehow it seems to not matter much except for the case with copies stored in changesets. Thanks to Kyle Lippincott for doing all the debugging and identifying the fix for this issue. Differential Revision: https://phab.mercurial-scm.org/D12573

tests: demonstrate crash on partial amend with copies in changesets See the fix in the next patch for explanation. Differential Revision: https://phab.mercurial-scm.org/D12572