deltas: Implement rollsums

This does an rsync-style prepared delta basically.  On my test data,
it shaves ~6MB of uncompressed data.  Not a huge amount, but I expect
this to be more useful for things like binaries which embed data, etc.

src/libostree/ostree-repo-static-delta-private.h

@@ -135,7 +135,8 @@ typedef enum {
   OSTREE_STATIC_DELTA_OP_OPEN_SPLICE_AND_CLOSE = 'S',
   OSTREE_STATIC_DELTA_OP_OPEN = 'o',
   OSTREE_STATIC_DELTA_OP_WRITE = 'w',
-  OSTREE_STATIC_DELTA_OP_SET_READ_SOURCE = 'R',
+  OSTREE_STATIC_DELTA_OP_SET_READ_SOURCE = 'r',
+  OSTREE_STATIC_DELTA_OP_UNSET_READ_SOURCE = 'R',
   OSTREE_STATIC_DELTA_OP_CLOSE = 'c'
 } OstreeStaticDeltaOpCode;