repo: Rename ostree_repo_check to ostree_repo_open

As it more clearly describes what the function does: load the
repo from disk and initialize it.

At the same time, add a cancellable parameter.

https://bugzilla.gnome.org/show_bug.cgi?id=707582

src/libostree/ostree-repo.h

@@ -56,7 +56,9 @@ OstreeRepo* ostree_repo_new (GFile *path);
 
 OstreeRepo* ostree_repo_new_default (void);
 
-gboolean      ostree_repo_check (OstreeRepo  *self, GError **error);
+gboolean      ostree_repo_open   (OstreeRepo     *self,
+                                  GCancellable   *cancellable,
+                                  GError        **error);
 
 gboolean      ostree_repo_create (OstreeRepo     *self,
                                   OstreeRepoMode  mode,