Bad behavior of GEODIFF_createRebasedChangesetEx if one input changeset is empty

[wonder-sk]

If one of the input diffs to GEODIFF_createRebasedChangesetEx() is empty, the file copy does a wrong thing:

https://github.com/MerginMaps/geodiff/blob/master/geodiff/src/geodiffrebase.cpp#L623

filecopy() has order of arguments 1. to, 2. from, but the code uses the opposite direction.

A regression from cf32f8a where boost::filesystem::copy() uses order of arguments 1. from, 2. to

Fortunately this should not affect ordinary mobile app / plugin users, as those call GEODIFF_rebase() which exists early if one of the input diffs is empty, without doing rebase.