ostree: Add a "remote refs" command

Works like "ostree refs" but fetches refs from a remote repo.

This depends on the remote repo having a summary file, but any repo
being served over HTTP *ought* to have one.

doc/ostree-remote.xml

@@ -63,6 +63,9 @@ Boston, MA 02111-1307, USA.
             <cmdsynopsis>
                 <command>ostree remote gpg-import</command> <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="req">NAME</arg> <arg choice="opt" rep="repeat">KEY-ID</arg>
             </cmdsynopsis>
+            <cmdsynopsis>
+                <command>ostree remote refs</command> <arg choice="req">NAME</arg>
+            </cmdsynopsis>
     </refsynopsisdiv>
 
     <refsect1>