lib/config: Rename change-update-summary to auto-...

Mildly bikeshed, though I find the name `auto-update-summary` to be
easier to grok than `change-update-summary`. I think it's because it can
be read as "verb-verb-noun" rather than "noun-verb-noun".

Closes: #1693
Approved by: mwleeds

src/libostree/ostree-repo-commit.c

@@ -2202,7 +2202,7 @@ ostree_repo_commit_transaction (OstreeRepo                  *self,
       return FALSE;
   g_clear_pointer (&self->txn.collection_refs, g_hash_table_destroy);
 
-  /* Update the summary if change-update-summary is set, because doing so was
+  /* Update the summary if auto-update-summary is set, because doing so was
    * delayed for each ref change during the transaction.
    */
   if (!_ostree_repo_maybe_regenerate_summary (self, cancellable, error))