`emms-source-file-default-directory' is currently the only completion
for calling `emms-add-...'.
My recent patch will replace `emms-source-file-default-directory' as the
completion with the directory of the track at point if:
* we are in a playlist buffer
* there is a track under point, and that track is a file
* the new variable `emms-source-file-directory-hint-p' is t (now the
I hope the reasoning behind it is clear: playlists probably contain
related tracks, and related tracks are probably stored near one another
in the file system. Hopefully this will get the user a bit closer to the
location of the next file they want to add.