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
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate