context: rewrite renamed() in terms of new copysource() where appropriate
It's only the filectx for committed files that have the copy source's
nodeid easily available; workingfilectx's and overlayfilectx's
renamed() simply find the nodeid by looking in the changeset's p1's
manifest.
Differential Revision: https://phab.mercurial-scm.org/D6009
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"base": "hg:.^"
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