give useful errors for file/directory transitions (issues/7)

CVS cannot handle a path changing between a file and a directory;
because it cannot handle a path being both a file and a directory
in even non-overlapping versions, as it has no way to version
directories per se.

Provide a useful error message at any point that a file/directory
transition exists between Git and CVS when exporting from Git to CVS.

Fixes #7