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-rollsum.h

@@ -0,0 +1,44 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2015 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+typedef struct {
+  GHashTable *from_rollsums;
+  GHashTable *to_rollsums;
+  guint crcmatches;
+  guint bufmatches;
+  guint total;
+  guint64 match_size;
+  GPtrArray *matches;
+} OstreeRollsumMatches;
+
+OstreeRollsumMatches *
+_ostree_compute_rollsum_matches (GBytes                           *from,
+                                 GBytes                           *to);
+
+void _ostree_rollsum_matches_free (OstreeRollsumMatches *rollsum);
+
+G_END_DECLS
+