ostree/summary: Generate an ostree-metadata ref when updating summary

This is the new way of publishing repository metadata, rather than as
additional-metadata in the summary file. The use of an ostree-metadata
ref means that the metadata from multiple upstream collections is not
conflated when doing P2P mirroring of many repositories.

The new ref is only generated if the repository has a collection ID set.
The old summary file continues to be generated for backwards
compatibility (and because it continues to be the canonical ref →
checksum map for the repository).

The new code is only used if configured with --enable-experimental-api.

Includes unit tests.

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

Closes: #1158
Approved by: cgwalters

man/ostree-summary.xml

@@ -83,7 +83,16 @@ Boston, MA 02111-1307, USA.
                   your organisation or repository using a dot prefix. The values
                   must be in GVariant text format. For example,
                   <command>exampleos.end-of-life "@t 1445385600"</command>.
-                </para></listitem>
+                </para>
+
+                <!-- FIXME: Uncomment this when collection ID support becomes non-experimental.
+                <para>If the repository has a collection ID configured, the
+                  <filename>ostree-metadata</filename> branch for that collection ID
+                  will also be updated with a new commit containing the given metadata,
+                  which will be signed if the summary file is signed.</para>
+                -->
+
+                </listitem>
             </varlistentry>
 
             <varlistentry>