grep: cast printf %.*s "precision" argument explicitly to int

grep: cast printf %.*s "precision" argument explicitly to int

On some systems, regoff_t that is the type of rm_so/rm_eo members are
wider than int; %.*s precision specifier expects an int, so use an explicit
cast.

A breakage reported on Darwin by Brian Gernhardt should be fixed with
this patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>