multi-pack-index: add 'write' verb

In anticipation of writing multi-pack-indexes, add a skeleton
'git multi-pack-index write' subcommand and send the options to a
write_midx_file() method. Also create a skeleton test script that
tests the 'write' subcommand.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -0,0 +1,7 @@
+#include "cache.h"
+#include "midx.h"
+
+int write_midx_file(const char *object_dir)
+{
+	return 0;
+}