upgrade: Add --override-commit=CHECKSUM option

Adds an entry to the origin file to force the OstreeSysrootUpgrader to
pull and deploy the given checksum.

  [origin]
  override-commit=CHECKSUM

If the option is not given, any such entry is explicitly removed from
the origin file to ensure we upgrade to the latest available commit.

doc/ostree-admin-upgrade.xml

@@ -88,6 +88,14 @@ Boston, MA 02111-1307, USA.
                     Permit deployment of chronologically older trees.
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--override-commit</option>="CHECKSUM"</term>
+
+                <listitem><para>
+                    Deploy CHECKSUM instead of the latest tree.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>