fsck: add argument --add-tombstones

It is used to create tombstones for missing commits

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

doc/ostree-fsck.xml

@@ -78,6 +78,13 @@ Boston, MA 02111-1307, USA.
                    Remove corrupted objects.
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--add-tombstones</option></term>
+                <listitem><para>
+                   Add tombstone commit for referenced but missing commits.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>