git: update to v2.15.1

Update to git version v2.15.1: With commit 0abe14f6 prepare_packed_git()
moved to packfile.[ch].

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: John Keeping <john@keeping.me.uk>

ui-clone.c

@@ -11,6 +11,7 @@
 #include "ui-clone.h"
 #include "html.h"
 #include "ui-shared.h"
+#include "packfile.h"
 
 static int print_ref_info(const char *refname, const struct object_id *oid,
                           int flags, void *cb_data)