Unify write_index_file functions

This patch unifies the write_index_file functions in
builtin-pack-objects.c and index-pack.c.  As the name
"index" is overloaded in git, move in the direction of
using "idx" and "pack idx" when refering to the pack index.
There should be no change in functionality.

Signed-off-by: Geert Bosch <[email protected]>
Acked-by: Nicolas Pitre <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Geert Bosch