Fix file importing code to not generate some duplicate directories

Fix file importing code to not generate some duplicate directories

build_file_tree() depends on the fact that the contents of a directory
are sorted immediately after it in the file list, but '.' sorts before '/'
lexicographically, so you'd actually end up with, eg:

    /etc/yum
    /etc/yum.conf
    /etc/yum/pluginconf.d

which would cause a second entry for /etc/yum to be added to the file
list.

For now I've fixed this by make compare_filenames() do a special strcmp
by hand, manually sorting '/' before anything else.