filter-repo: fix naming to be less redundant

'tag.tag_message' is redundant.  Simplify it to just 'tag.message'.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -667,7 +667,7 @@ class Tag(_GitElement):
     self.tagger_date  = tagger_date
 
     # Store the tag message
-    self.tag_message = tag_msg
+    self.message = tag_msg
 
   def dump(self, file_):
     """
@@ -683,7 +683,7 @@ class Tag(_GitElement):
       file_.write('tagger %s <%s> ' % (self.tagger_name, self.tagger_email))
       file_.write(self.tagger_date)
       file_.write('\n')
-    file_.write('data %d\n%s' % (len(self.tag_message), self.tag_message))
+    file_.write('data %d\n%s' % (len(self.message), self.message))
     file_.write('\n')
 
 class Progress(_GitElement):