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.

src/libostree/ostree-repo-private.h

@@ -63,6 +63,7 @@ G_BEGIN_DECLS
 /* Well-known keys for the additional metadata field in a commit in a ref entry
  * in a summary file. */
 #define OSTREE_COMMIT_TIMESTAMP "ostree.commit.timestamp"
+#define OSTREE_COMMIT_VERSION "ostree.commit.version"
 
 typedef enum {
   OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0),