`git config --list` error on value-less flags

[fmigneault]

If a git config contains a "value-less" feature flag mapping to gitattributes, such as required, it fails invocation when parsing the result:

...
  File ".../git-filter-repo/git-filter-repo", line 1695, in get_config_settings
    return dict(item.split(b'\n', maxsplit=1)
ValueError: dictionary update sequence element #35 has length 1; 2 is required

(note: above includes latest fixes including #684, it is a different error, although it looks similar)

As shown by the git docs:

• https://git-scm.com/docs/git-config#Documentation/git-config.txt-filterdriverclean
• https://git-scm.com/docs/gitattributes#_filter

[filter "lfs"]
  clean = "... clean command ..."
  required