PICARD-1723: Consider disc number when comparing metadata

Improves results when matching files to multi-disc albums.

picard/metadata.py

@@ -123,6 +123,8 @@ class Metadata(MutableMapping):
         ('album', 12),
         ('tracknumber', 6),
         ('totaltracks', 5),
+        ('discnumber', 5),
+        ('totaldiscs', 4),
     ]
 
     multi_valued_joiner = MULTI_VALUED_JOINER
@@ -172,7 +174,7 @@ class Metadata(MutableMapping):
             a = self[name]
             b = other[name]
             if a and b:
-                if name in ('tracknumber', 'totaltracks'):
+                if name in ('tracknumber', 'totaltracks', 'discnumber', 'totaldiscs'):
                     try:
                         ia = int(a)
                         ib = int(b)

test/test_metadata.py

@@ -280,7 +280,20 @@ class MetadataTest(PicardTestCase):
         m1["tracknumber"] = "1"
         m2 = Metadata()
         m2["tracknumber"] = "2"
+        m3 = Metadata()
+        m3["tracknumber"] = "2"
         self.assertEqual(m1.compare(m2), 0)
+        self.assertEqual(m2.compare(m3), 1)
+
+    def test_compare_discnumber_difference(self):
+        m1 = Metadata()
+        m1["discnumber"] = "1"
+        m2 = Metadata()
+        m2["discnumber"] = "2"
+        m3 = Metadata()
+        m3["discnumber"] = "2"
+        self.assertEqual(m1.compare(m2), 0)
+        self.assertEqual(m2.compare(m3), 1)
 
     def test_compare_deleted(self):
         m1 = Metadata()