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
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown