merge: avoid superfluous filemerges when grafting through renames (issue5407) This is a fix for a regression introduced by the patches for issue4028. The test changes are due to us doing fewer _checkcopies searches now, which makes some test outputs revert to the pre-issue4028 behavior. That issue itself remains fixed, we only skip copy tracing for files where it isn't relevant. As a nice side effect, this makes copy detection much faster when tracing backwards through lots of renames.