compat: temporarily move copies fix to compat.py
This patch moves a fixed version of copies._fullcopytracing() to compat.py as
this was not fixed in core before release and also we need to do this anyway for
the older versions of mercurial.
This fix is that when base changeset is not ancestor of any of the merging
commits, we don't throw an error.
The phabricator differential which will fix this in core is
https://phab.mercurial-scm.org/D3896.
This fixes the traceback we get in previous changeset while relocating one of
the content-divergent changeset.