ValueError while cloning git@github.com:getpelican/pelican-themes.git

$ hg clone git@github.com:getpelican/pelican-themes.git        
destination directory: pelican-themes
importing git objects into hg
** unknown exception encountered, please report by visiting                     
** https://mercurial-scm.org/wiki/BugTracker
** Python 3.9.0 | packaged by conda-forge | (default, Nov 26 2020, 07:57:39) [GCC 9.3.0]
** Mercurial Distributed SCM (version 5.6)
** Extensions loaded: churn, extdiff, hggit, rebase, absorb, evolve, topic, purge, hgfluiddyn, graphlog
Traceback (most recent call last):
  File "/data0/opt/miniconda3/envs/_env_mercurial/bin/hg", line 43, in <module>
    dispatch.run()
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 113, in run
    status = dispatch(req)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 303, in dispatch
    ret = _runcatch(req) or 0
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 479, in _runcatch
    return _callcatch(ui, _runcatchfunc)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 488, in _callcatch
    return scmutil.callcatch(ui, func)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/scmutil.py", line 153, in callcatch
    return func()
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 469, in _runcatchfunc
    return _dispatch(req)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 1232, in _dispatch
    return runcommand(
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 917, in runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 1244, in _runcommand
    return cmdfunc()
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/dispatch.py", line 1230, in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **strcmdopt)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/util.py", line 1867, in check
    return func(*args, **kwargs)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/commands.py", line 1895, in clone
    r = hg.clone(
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/mercurial/hg.py", line 928, in clone
    exchange.pull(
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/hggit/util.py", line 63, in inner
    return f(*args, **kwargs)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/hggit/__init__.py", line 563, in exchangepull
    pullop.cgresult = repo.githandler.fetch(remote.path, heads)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/hggit/git_handler.py", line 308, in fetch
    imported = self.import_git_objects(remote_name, filteredrefs)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/hggit/git_handler.py", line 850, in import_git_objects
    self.import_git_commit(commit)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/hggit/git_handler.py", line 924, in import_git_commit
    gitmodules = self.parse_gitmodules(git_commit_tree)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/hggit/git_handler.py", line 1738, in parse_gitmodules
    tt = dul_config.ConfigFile.from_file(fo)
  File "/data0/opt/miniconda3/envs/_env_mercurial/lib/python3.9/site-packages/dulwich/config.py", line 439, in from_file
    raise ValueError("invalid variable name %r" % setting)
ValueError: invalid variable name b'<<<<<<< HEAD'

I can reproduce with 0.10.4 and with the 1.0.x branch.