lib/repo: Add an API to get min-free-space-* reserved bytes

https://phabricator.endlessm.com/T23694

Closes: #1715
Approved by: cgwalters

src/libostree/ostree-repo.h

@@ -127,6 +127,8 @@ gboolean      ostree_repo_equal (OstreeRepo *a,
 _OSTREE_PUBLIC
 OstreeRepoMode ostree_repo_get_mode (OstreeRepo  *self);
 
+_OSTREE_PUBLIC
+guint64        ostree_repo_get_min_free_space_bytes (OstreeRepo *self);
 _OSTREE_PUBLIC
 GKeyFile *    ostree_repo_get_config (OstreeRepo *self);