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: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>