lib/repo: Add commit version metadata to summary metadata

The commit metadata `version` key is well established but getting it for
a remote commit is cumbersome since the commit object needs to be
fetched and loaded. Including it in the summary additional metadata
allows a much more convenient view of what each of the remote refs
represents.

man/ostree-summary.xml

@@ -183,6 +183,7 @@ License along with this library. If not, see <https://www.gnu.org/licenses/>.
     Latest Commit (4.2 MB):
       9828ab80f357459b4ab50f0629beab2ae3b67318fc3d161d10a89fae353afa90
     Timestamp (ostree.commit.timestamp): 2017-11-21T01:41:10-08
+    Version (ostree.commit.version): 1.2.3
 
 Last-Modified (ostree.summary.last-modified): 2018-01-12T22:06:38-08
 </programlisting>