config: add new parameter "commit-update-summary" to core section

When set to true, the summary file is automatically updated after
a commit.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

doc/ostree.repo-config.xml

@@ -84,6 +84,13 @@ 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>fsync</varname></term>
         <listitem><para>Boolean value controlling whether or not to