lib/config: Deprecate commit-update-summary option

Now that we have `auto-update-summary`, there is no point in having
`commit-update-summary`. The latter also only had an effect through
the `commit` CLI command, whereas the former is embedded directly in
libostree.

There is one corner case that slips through: `commit` would update the
summary file even if orphan commits were created, which we no longer do
here. I can't imagine anyone relying on this, so it seems safe to drop.

Closes: #1689

Closes: #1693
Approved by: mwleeds

man/ostree.repo-config.xml

@@ -86,23 +86,23 @@ Boston, MA 02111-1307, USA.
         <listitem><para>Currently, this must be set to <literal>1</literal>.</para></listitem>
       </varlistentry>
 
-      <varlistentry>
-        <term><varname>commit-update-summary</varname></term>
-        <listitem><para>Boolean value controlling whether or not to
-        automatically update the summary file after a commit.  Defaults
-        to <literal>false</literal>.</para></listitem>
-      </varlistentry>
-
       <varlistentry>
         <term><varname>auto-update-summary</varname></term>
         <listitem><para>Boolean value controlling whether or not to
         automatically update the summary file after any ref is added,
-        removed, or updated. This covers a superset of the cases covered by
-        commit-update-summary, with the exception of orphan commits which
-        shouldn't affect the summary anyway. Defaults to <literal>false</literal>.
+        removed, or updated. Other modifications which may render a
+        summary file stale (like static deltas, or collection IDs) do
+        not currently trigger an auto-update.
         </para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>commit-update-summary</varname></term>
+        <listitem><para>This option is deprecated. Use
+        <literal>auto-update-summary</literal> instead, for which this
+        option is now an alias.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>fsync</varname></term>
         <listitem><para>Boolean value controlling whether or not to