man/prune, bash: Add --commit-only flag for ostree prune

Update the man page and the auto-complete script
to include the --commit-only flag

bash/ostree

@@ -805,6 +805,7 @@ _ostree_prune() {
         --no-prune
         --refs-only
         --static-deltas-only
+        --commit-only
     "
 
     local options_with_args="

man/ostree-prune.xml

@@ -120,6 +120,17 @@ License along with this library. If not, see <https://www.gnu.org/licenses/>.
                   <option>--delete-commit</option>.
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--commit-only</option></term>
+
+                <listitem><para>
+                    Only traverse and delete commit objects. This leaves orphaned meta and 
+                    content objects, which can be cleaned up with another prune invocation.
+                    One may want to use this option to cheaply delete multiple commits,
+                    and then clean up with a more expensive prune at the end.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>