Add test-scrap-cache-tree

A simple utility that invalidates all existing cache-tree data.  We
need this for tests.  (We don't need a tool to rebuild the cache-tree
data; git read-tree HEAD works for that.)

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-scrap-cache-tree.c

@@ -0,0 +1,17 @@
+#include "cache.h"
+#include "tree.h"
+#include "cache-tree.h"
+
+static struct lock_file index_lock;
+
+int main(int ac, char **av)
+{
+	int fd = hold_locked_index(&index_lock, 1);
+	if (read_cache() < 0)
+		die("unable to read index file");
+	active_cache_tree = NULL;
+	if (write_cache(fd, active_cache, active_nr)
+	    || commit_lock_file(&index_lock))
+		die("unable to write index file");
+	return 0;
+}