bin/refs: Allow overwriting existing ref

Currently if you want to update a non-alias ref, you need to first check
if it exists and use either `ostree refs --create` or `ostree reset` as
appropriate. That's unnecessarily complicated and is much less
convenient than the old `write-refs` builtin that simply called
`ostree_repo_set_ref_immediate()` without any checks.

Add a `--force` option to be used with `--create` that does not raise an
error when the destination ref already exists.

Closes: #1870
Approved by: jlebon

man/ostree-refs.xml

@@ -126,6 +126,16 @@ Boston, MA 02111-1307, USA.
                   PREFIX are deleted.
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--force</option></term>
+
+                <listitem><para>
+                  When creating <literal>NEWREF</literal> with
+                  <option>--create</option>, allow an existing ref to be
+                  updated instead of erroring.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>