lib/repo: Add ostree_repo_equal() for comparing repos

This will compare their root directory inodes to see if they are the
same repository on disk. A convenience method for the users of the
public API who can’t access OstreeRepo.inode.

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

Closes: #1179
Approved by: cgwalters

apidoc/ostree-sections.txt

@@ -287,6 +287,7 @@ ostree_repo_get_path
 ostree_repo_get_mode
 ostree_repo_get_config
 ostree_repo_get_dfd
+ostree_repo_equal
 ostree_repo_copy_config
 ostree_repo_remote_add
 ostree_repo_remote_delete