commit: add --timestamp=TIMESTAMP

It allows to override the timestamp of the commit.

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

doc/ostree-commit.xml

@@ -198,6 +198,14 @@ Boston, MA 02111-1307, USA.
                     GPG home directory to use when looking for keyrings (if have GPGME - GNU Privacy Guard Made Easy).
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--timestamp</option>="TIMESTAMP"</term>
+
+                <listitem><para>
+                    Override the timestamp of the commit to TIMESTAMP.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>