filter-repo (python3): more flush()ing needed under python3

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

git-filter-repo

@@ -1655,6 +1655,7 @@ class FastExportFilter(object):
                                                    stdout=subprocess.PIPE,
                                                    cwd=self._repo_working_dir)
           batch_check_process.stdin.write(refname+"\n")
+          batch_check_process.stdin.flush()
           line = batch_check_process.stdout.readline()
           m = batch_check_output_re.match(line)
           if not m or m.group(2) != 'tag':