Write musicbrainz_discid tag when disc is unknown

Fixes #280.

whipper/command/cd.py

@@ -399,7 +399,7 @@ Log files will log the path to tracks relative to this directory.
                                               offset=int(self.options.offset),
                                               device=self.device,
                                               taglist=self.program.getTagList(
-                                                  number),
+                                                  number, self.mbdiscid),
                                               overread=self.options.overread,
                                               what='track %d of %d%s' % (
                                                   number,

whipper/common/program.py

@@ -401,7 +401,7 @@ class Program:
         self._stdout.write('\n')
         return ret
 
-    def getTagList(self, number):
+    def getTagList(self, number, mbdiscid):
         """
         Based on the metadata, get a dict of tags for the given track.
 
@@ -421,7 +421,6 @@ class Program:
             disc = self.metadata.title
             mbidAlbum = self.metadata.mbid
             mbidTrackAlbum = self.metadata.mbidArtist
-            mbDiscId = self.metadata.discid
 
             if number > 0:
                 try:
@@ -439,6 +438,9 @@ class Program:
 
         tags = {}
 
+        if number > 0:
+            tags['MUSICBRAINZ_DISCID'] = mbdiscid
+
         if self.metadata and not self.metadata.various:
             tags['ALBUMARTIST'] = albumArtist
         tags['ARTIST'] = trackArtist
@@ -456,7 +458,6 @@ class Program:
                 tags['MUSICBRAINZ_ARTISTID'] = mbidTrackArtist
                 tags['MUSICBRAINZ_ALBUMID'] = mbidAlbum
                 tags['MUSICBRAINZ_ALBUMARTISTID'] = mbidTrackAlbum
-                tags['MUSICBRAINZ_DISCID'] = mbDiscId
 
         # TODO/FIXME: ISRC tag