core: Import bup's "rollsum" code, add a test case

For static deltas, one strategy that will be employed is to split each
object into chunks, and only include changed chunks in the deltas.

src/libostree/bupsplit.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2011 Avery Pennarun. All rights reserved.
+ * 
+ * (This license applies to bupsplit.c and bupsplit.h only.)
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ * 
+ *    1. Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ * 
+ *    2. Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in
+ *       the documentation and/or other materials provided with the
+ *       distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY AVERY PENNARUN ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+#ifndef __BUPSPLIT_H
+#define __BUPSPLIT_H
+
+#define BUP_BLOBBITS (13)
+#define BUP_BLOBSIZE (1<<BUP_BLOBBITS)
+#define BUP_WINDOWBITS (7)
+#define BUP_WINDOWSIZE (1<<(BUP_WINDOWBITS-1))
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+    
+int bupsplit_find_ofs(const unsigned char *buf, int len, int *bits);
+int bupsplit_selftest(void);
+
+#ifdef __cplusplus
+}
+#endif
+    
+#endif /* __BUPSPLIT_H */