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
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a