add context pointer to read_tree_recursive()

add context pointer to read_tree_recursive()

Add a pointer parameter to read_tree_recursive(), which is passed to the
callback function.  This allows callers of read_tree_recursive() to
share data with the callback without resorting to global variables.  All
current callers pass NULL.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>