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

src/libostree/ostree-repo.h

@@ -123,6 +123,8 @@ GFile *       ostree_repo_get_path (OstreeRepo  *self);
 _OSTREE_PUBLIC
 int           ostree_repo_get_dfd (OstreeRepo  *self);
 
+_OSTREE_PUBLIC
+guint         ostree_repo_hash (OstreeRepo *self);
 _OSTREE_PUBLIC
 gboolean      ostree_repo_equal (OstreeRepo *a,
                                  OstreeRepo *b);