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>

.gitignore

@@ -171,6 +171,7 @@
 /test-date
 /test-delta
 /test-dump-cache-tree
+/test-scrap-cache-tree
 /test-genrandom
 /test-index-version
 /test-line-buffer

Makefile

@@ -435,6 +435,7 @@ TEST_PROGRAMS_NEED_X += test-ctype
 TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
+TEST_PROGRAMS_NEED_X += test-scrap-cache-tree
 TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-index-version
 TEST_PROGRAMS_NEED_X += test-line-buffer

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;
+}