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.

tests/test-summary-view.sh

@@ -56,6 +56,7 @@ assert_file_has_content_literal summary.txt "* main"
 assert_file_has_content_literal summary.txt "* other"
 assert_file_has_content_literal summary.txt "ostree.summary.last-modified"
 assert_file_has_content_literal summary.txt "Timestamp (ostree.commit.timestamp): "
+assert_file_has_content_literal summary.txt "Version (ostree.commit.version): 3.2"
 echo "ok view summary"
 
 # Check the summary can be viewed raw too.