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