pack-bitmap.c: rename "idx_name" to "bitmap_name"

pack-bitmap.c: rename "idx_name" to "bitmap_name"

In "open_pack_bitmap_1()" and "open_midx_bitmap_1()" we use
a var named "idx_name" to represent the bitmap filename which
is computed by "midx_bitmap_filename()" or "pack_bitmap_filename()"
before we open it.

There may bring some confusion in this "idx_name" naming, which
might lead us to think of ".idx "or" multi-pack-index" files,
although bitmap is essentially can be understood as a kind of index,
let's define this name a little more accurate here.

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>