I wanted to test the fix and I got the following error around the additional `name.decode(...)` change:

$ brz log
brz: ERROR: AttributeError: 'str' object has no attribute 'decode'

Traceback (most recent call last):
  File "/home/neil/proj/brz/brz.dev/breezy/commands.py", line 1009, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/neil/proj/brz/brz.dev/breezy/commands.py", line 1194, in run_bzr
    ret = run(*run_argv)
  File "/home/neil/proj/brz/brz.dev/breezy/commands.py", line 760, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/neil/proj/brz/brz.dev/breezy/commands.py", line 784, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/neil/proj/brz/brz.dev/breezy/cleanup.py", line 136, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/neil/proj/brz/brz.dev/breezy/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/neil/proj/brz/brz.dev/breezy/commands.py", line 1211, in ignore_pipe
    result = func(*args, **kwargs)
  File "/home/neil/proj/brz/brz.dev/breezy/builtins.py", line 2874, in run
    Logger(b, rqst).show(lf)
  File "/home/neil/proj/brz/brz.dev/breezy/log.py", line 393, in show
    self._show_body(lf)
  File "/home/neil/proj/brz/brz.dev/breezy/log.py", line 421, in _show_body
    for lr in generator.iter_log_revisions():
  File "/home/neil/proj/brz/brz.dev/breezy/log.py", line 463, in iter_log_revisions
    revision_iterator = self._create_log_revision_iterator()
  File "/home/neil/proj/brz/brz.dev/breezy/log.py", line 515, in _create_log_revision_iterator
    self.rqst.get('end_revision'))
  File "/home/neil/proj/brz/brz.dev/breezy/log.py", line 1100, in _get_revision_limits
    end_revno = branch.revno()
  File "/home/neil/proj/brz/brz.dev/breezy/branch.py", line 969, in revno
    return self.last_revision_info()[0]
  File "/home/neil/proj/brz/brz.dev/breezy/branch.py", line 986, in last_revision_info
    self._last_revision_info_cache = self._read_last_revision_info()
  File "/home/neil/proj/brz/brz.dev/breezy/git/branch.py", line 638, in _read_last_revision_info
    last_revid = self.last_revision()
  File "/home/neil/proj/brz/brz.dev/breezy/git/branch.py", line 539, in last_revision
    return self.lookup_foreign_revision_id(self.head)
  File "/home/neil/proj/brz/brz.dev/breezy/git/branch.py", line 548, in lookup_foreign_revision_id
    self.mapping)
  File "/home/neil/proj/brz/brz.dev/breezy/git/repository.py", line 475, in lookup_foreign_revision_id
    commit = self._git.object_store.peel_sha(foreign_revid)
  File "/home/neil/.asdf/installs/python/3.6.5/lib/python3.6/site-packages/dulwich/object_store.py", line 256, in peel_sha
    obj = self[sha]
  File "/home/neil/.asdf/installs/python/3.6.5/lib/python3.6/site-packages/dulwich/object_store.py", line 119, in __getitem__
    type_num, uncomp = self.get_raw(sha)
  File "/home/neil/.asdf/installs/python/3.6.5/lib/python3.6/site-packages/dulwich/object_store.py", line 446, in get_raw
    for pack in self.packs:
  File "/home/neil/proj/brz/brz.dev/breezy/git/transportgit.py", line 594, in packs
    self._update_pack_cache()
  File "/home/neil/proj/brz/brz.dev/breezy/git/transportgit.py", line 598, in _update_pack_cache
    for pack in self._load_packs():
  File "/home/neil/proj/brz/brz.dev/breezy/git/transportgit.py", line 625, in _load_packs
    name = name.decode(sys.getfilesystemencoding())
AttributeError: 'str' object has no attribute 'decode'

brz 3.0a1 on python 3.6.5 (Linux-4.15.0-34-generic-x86_64-with-debian-buster-
    sid)
arguments: ['/home/neil/usr/bin/brz', 'log']
plugins: bash_completion[3.0a1], changelog_merge[3.0a1],
    commitfromnews[unknown], cvs[3.0a1], darcs[unknown], email[unknown],
    fastimport[unknown], grep[3.0a1], launchpad[3.0a1], mtn[3.0a1],
    netrc_credential_store[3.0a1], news_merge[3.0a1], po_merge[3.0a1],
    repodebug[unknown], stats[3.0a1], upload[3.0a1], weave_fmt[3.0a1]
encoding: 'utf-8', fsenc: 'utf-8', lang: 'en_US.UTF-8'

*** Bazaar has encountered an internal error.  This probably indicates a
    bug in Bazaar.  You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/brz/+filebug
    including this traceback and a description of the problem.

I tested by doing `brz merge lp:~jelmer/brz/alpha-fix-git-python3` and then `brz log` in the same git repository as the original bug report.