filter-repo: make tagger optional

Apparently, despite the git-fast-import.txt documentation, tagger is
optional for both fast-export and fast-import.  I suspect this is
because there are several (old) tags in the linux.git repository that
have no tagger, so if we want to test rewriting linux.git history then
we need to make it optional for filter-repo too.

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

git-filter-repo

@@ -687,9 +687,10 @@ class Tag(_GitElement):
     file_.write('tag %s\n' % self.ref)
     mark = ':' if isinstance(self.from_ref, int) else ''
     file_.write('from {}{}\n'.format(mark, self.from_ref))
-    file_.write('tagger %s <%s> ' % (self.tagger_name, self.tagger_email))
-    _write_date(file_, self.tagger_date)
-    file_.write('\n')
+    if self.tagger_name:
+      file_.write('tagger %s <%s> ' % (self.tagger_name, self.tagger_email))
+      _write_date(file_, self.tagger_date)
+      file_.write('\n')
     file_.write('data %d\n%s' % (len(self.tag_message), self.tag_message))
     file_.write('\n')
 
@@ -1382,7 +1383,9 @@ class FastExportFilter(object):
     if self._currentline.startswith('original-oid'):
       original_id = self._parse_original_id();
 
-    (tagger_name, tagger_email, tagger_date) = self._parse_user('tagger')
+    tagger_name, tagger_email, tagger_date = None, None, None
+    if self._currentline.startswith('tagger'):
+      (tagger_name, tagger_email, tagger_date) = self._parse_user('tagger')
     tag_msg = self._parse_data()
     if self._currentline == '\n':
       self._advance_currentline()