See e.g. https://ci.chromium.org/p/infra/builders/luci.infra.cron/wpt-importer/b8928784556751989472 and https://ci.chromium.org/p/infra/builders/luci.infra.cron/wpt-exporter/b8928783566886451152:

  Traceback (most recent call last):
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/wpt_import.py", line 26, in <module>
      main()
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/wpt_import.py", line 19, in main
      host.exit(importer.main())
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/test_importer.py", line 131, in main
      commits = self.apply_exportable_commits_locally(local_wpt)
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/test_importer.py", line 337, in apply_exportable_commits_locally
      commits = self.exportable_but_not_exported_commits(local_wpt)
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/test_importer.py", line 368, in exportable_but_not_exported_commits
      self.host, local_wpt, self.wpt_github, require_clean=False, verify_merged_pr=True)
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/chromium_exportable_commits.py", line 47, in exportable_commits_over_last_n_commits
      return _exportable_commits_since(start_commit, host, local_wpt, wpt_github, require_clean, verify_merged_pr)
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/chromium_exportable_commits.py", line 73, in _exportable_commits_since
      state, error = get_commit_export_state(commit, local_wpt, wpt_github, verify_merged_pr)
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/chromium_exportable_commits.py", line 106, in get_commit_export_state
      patch = chromium_commit.format_patch()
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/w3c/chromium_commit.py", line 121, in format_patch
      ] + filtered_files, cwd=self.absolute_chromium_dir)
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/common/system/executive.py", line 319, in run_command
      close_fds=self._should_close_fds())
    File "/b/swarming/w/ir/kitchen-workdir/src/third_party/blink/tools/blinkpy/common/system/executive.py", line 393, in popen
      return subprocess.Popen(string_args, **kwargs)
    File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/subprocess.py", line 390, in __init__
      errread, errwrite)
    File "/b/swarming/w/ir/cipd_bin_packages/lib/python2.7/subprocess.py", line 1025, in _execute_child
      raise child_exception
  OSError: [Errno 7] Argument list too long

The problem's that exportable_commits_over_last_n_commits() ends up calling "git rev-list HEAD~10001..HEAD --reverse -- /data/src/chromium/src/third_party/blink/web_tests/external/wpt/", which returns, among others, commit 77578ccb4082ae20a9326d9e673225f1189ebb63, which is the big one renaming everything. When we get to format_patch(), we're essentially passing all tests in WPT on the command-line.