deltas: Add and document no-deltas-in-summary config option

By default this is FALSE to keep existing clients working.

man/ostree.repo-config.xml

@@ -249,6 +249,20 @@ Boston, MA 02111-1307, USA.
         costly).
         </para></listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>no-deltas-in-summary</varname></term>
+        <listitem><para>Boolean value controlling whether OSTree should skip
+        putting an index of available deltas in the summary file. Defaults to false.
+        </para>
+        <para>
+        Since 2020.7 OSTree can use delta indexes outside the summary file,
+        making the summary file smaller (especially for larger repositories). However
+        by default we still create the index in the summary file to make older clients
+        work. If you know all clients will be 2020.7 later you can enable this to
+        save network bandwidth.
+        </para></listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>
 

src/libostree/ostree-repo.c

@@ -5749,6 +5749,8 @@ ostree_repo_regenerate_summary (OstreeRepo     *self,
    * commits from working.
    */
   g_autoptr(OstreeRepoAutoLock) lock = NULL;
+  gboolean no_deltas_in_summary = FALSE;
+
   lock = _ostree_repo_auto_lock_push (self, OSTREE_REPO_LOCK_EXCLUSIVE,
                                       cancellable, error);
   if (!lock)
@@ -5781,6 +5783,12 @@ ostree_repo_regenerate_summary (OstreeRepo     *self,
       }
   }
 
+  if (!ot_keyfile_get_boolean_with_default (self->config, "core",
+                                            "no-deltas-in-summary", FALSE,
+                                            &no_deltas_in_summary, error))
+    return FALSE;
+
+  if (!no_deltas_in_summary)
     {
       g_autoptr(GPtrArray) delta_names = NULL;
       g_auto(GVariantDict) deltas_builder = OT_VARIANT_BUILDER_INITIALIZER;