copies: avoid early return in _combine_changeset_copies
We have to change how we deal with matching (see next changeset) and that
processing is common. So we shuffle things around before doing the semantic
change for clarity.
Differential Revision: https://phab.mercurial-scm.org/D9584
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true