This is in preparation for keeping two entry lists in the
dir object.
This patch adds and uses the ALLOC_GROW() macro, which
implements the commonly used idiom of growing a dynamic
array using the alloc_nr function (not just in dir.c, but
everywhere).
We also move creation of a dir_entry to dir_entry_new.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>