convert: look up copies in getchanges instead of getcommit
svn: defer path expansion until getchanges to reduce latency, as well as memory
usage when converting incrementally.
convert: svn: pull up path to file expansion code into separate function.
This is part of the process for deferring path expansion until getchanges.
Copy detection also needs to be moved out of the commit object.
convert: svn: ensure leading / is removed from paths in _find_children (broken in
2bd996d0aaf8)