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.
adding da/foo
adding foo
% manifest
200 Script output follows
drwxr-xr-x da
-rw-r--r-- 4 foo
200 Script output follows
-rw-r--r-- 4 foo