lib/repo: Add ostree_repo_hash() and tests

Add a hash function for OstreeRepo instances, which relies on the repo
being open, and hence being able to hash the device and inode of its
root directory.

Add unit tests for this and ostree_repo_equal().

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

https://github.com/ostreedev/ostree/issues/1191

Closes: #1205
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_hash
 ostree_repo_equal
 ostree_repo_copy_config
 ostree_repo_remote_add