ostree/fsck: Add --verify-back-refs option

This new option verifies that the refs listed in the ref-bindings for
each commit all point to that commit (i.e. there aren’t multiple commits
listing the same ref in their ref-bindings, and there aren’t any commits
with non-empty ref-bindings which aren’t pointed at by a ref).

This is useful when generating a new repository from scratch, but not
useful when adding new commits to an existing repository (since the old
commits will still, correctly, have ref-bindings from when the refs
pointed at them). That’s why it has to be enabled explicitly using
--verify-back-refs, rather than being on by default.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1347
Approved by: cgwalters

man/ostree-fsck.xml

@@ -85,6 +85,17 @@ Boston, MA 02111-1307, USA.
                    Add tombstone commit for referenced but missing commits.
                 </para></listitem>
             </varlistentry>
+
+            <varlistentry>
+                <term><option>--verify-back-refs</option></term>
+                <listitem><para>
+                   Verify that all the refs listed in a commit’s ref-bindings
+                   point to that commit. This cannot be used in repositories
+                   where the target of refs is changed over time as new commits
+                   are added, but can be used in repositories which are
+                   regenerated from scratch for each commit.
+                </para></listitem>
+            </varlistentry>
         </variablelist>
     </refsect1>