Fix fragile parsing of bs1770gain output
Fix fragile parsing of bs1770gain output

bs1770gain is sorting the filenames internally, so in some corner cases it returns the results in a different order from the command-line arguments.

This patch uses the --xml option of bs1770gain (introduced in 0.4.6) in order to read the filenames reliably.

beetbox/beets,sampsyo/beets,ibmibmibm/beets,shamangeorge/beets,beetbox/beets,SusannaMaria/beets,shamangeorge/beets,beetbox/beets,sampsyo/beets,shamangeorge/beets,ibmibmibm/beets,beetbox/beets,sampsyo/beets,SusannaMaria/beets,jackwilsdon/beets,SusannaMaria/beets,ibmibmibm/beets,shamangeorge/beets,SusannaMaria/beets,jackwilsdon/beets,ibmibmibm/beets,jackwilsdon/beets,sampsyo/beets,jackwilsdon/beets