deltas: Add _ostree_get_relative_static_delta_index_path()

This gets the subpath for a delta index file, which is of the form
"delta-indexes/$commit.index", that contains all the deltas going
to the particular commit.

src/libostree/ostree-core-private.h

@@ -135,6 +135,9 @@ _ostree_get_relative_static_delta_part_path (const char        *from,
                                              const char        *to,
                                              guint              i);
 
+char *
+_ostree_get_relative_static_delta_index_path (const char        *to);
+
 static inline char * _ostree_get_commitpartial_path (const char *checksum)
 {
   return g_strconcat ("state/", checksum, ".commitpartial", NULL);