MNT: gitrepo: Update comment about '-c annex.largefiles' use

With older versions of git-annex, 'git add' added content to the
annex, as described in ad0158c (BF(v7): gitrepo: Avoid adding files
to annex, 2019-09-23).  git-annex no longer does so as of 7.20191024.

datalad/support/gitrepo.py

@@ -1186,7 +1186,9 @@ def add_(self, files, git=True, git_options=None, update=False):
             add_out = self._git_custom_command(
                 files,
                 # Set annex.largefiles to prevent storing files in annex when
-                # GitRepo() is instantiated with a v6+ annex repo.
+                # GitRepo() is instantiated with a v6+ annex repo with older
+                # git-annex versions. This should be removed when
+                # GIT_ANNEX_MIN_VERSION at least 7.20191024.
                 ['git', '-c', 'annex.largefiles=nothing', 'add'] +
                 assure_list(git_options) +
                 to_options(update=update) + ['--verbose']