copies: move short-circuiting of dirstate copies out of _forwardcopies()
I'd like to move the filtering of copies we do after chaining to the
end of all chaining (in a single place in pathcopies()). One problem
that came up when trying that was that we allow things like `hg cp -f
<file> <existing file>` so the user can later amend that in. Filtering
at the end would mean that we remove those copies. That would break
hg st -C. This patch therefore moves the short-circuiting of
dirstate copies into pathcopies() so we can more easily handle the
dirstate-only case differently.
I initially thought this might change some behavior when the user does
hg status --rev 'wdir()' --rev . during an uncommitted merge, since
_backwardrenames() would reverse the copies in that case. However, I
couldn't come up with a test case where it made a difference.
Differential Revision: https://phab.mercurial-scm.org/D6600