amend: fix amend with copies in extras If copy information is stored only in the commit extras and not in filelogs, then they get lost on amend if the file wasn't also modified in the working copy. That's because we create `filectx` object from the old commit in those cases, and the `.copysource()` of such objects read only from the filelog. This patch fixes it by always creating a new `memfilectx` in these cases, passing the calculated copy information to it. Differential Revision: https://phab.mercurial-scm.org/D12387

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