copies: fix crash on in changeset-centric tracing from commit to itself
When we trace copies from a changeset to itself, the "work" queue ends
up empty and we hit the "assert False" after it.
It was only the last of the three added tests that failed before this
patch. That is because the other two cases have fast paths, so
_committedforwardcopies() is never reached.
Differential Revision: https://phab.mercurial-scm.org/D6675
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors