cli/rev-parse: Add --single option

In the current "ostree native container" flow, we're inserting
a commit object into the repo but with no refs.

We have hacks in a few places to find the commit digest via e.g.
`find repo/objects -name *.commit` but that's a horrible hack.
Add `ostree rev-parse --single` which will print the single commit,
and error out if there is not exactly one commit.

Co-authored-by: Jonathan Lebon <jonathan@jlebon.com>

man/ostree-rev-parse.xml

@@ -54,6 +54,19 @@ License along with this library. If not, see <https://www.gnu.org/licenses/>.
     </refsynopsisdiv>
 
 
+    <refsect1>
+        <title>Options</title>
+        <variablelist>
+            <varlistentry>
+                <term><option>--single</option>, <option>-S</option></term>
+
+                <listitem><para>
+                    If the repository has exactly one commit, then print it; any other case will result in an error.
+                </para></listitem>
+            </varlistentry>
+        </variablelist>
+    </refsect1>
+
 <!-- Is this accurate for all cases?  This is what I observed -->
     <refsect1>
         <title>Description</title>