pack-bitmap.c: more aggressively free in free_bitmap_index()

pack-bitmap.c: more aggressively free in free_bitmap_index()

The function free_bitmap_index() is somewhat lax in what it frees. There
are two notable examples:

  - While it does call kh_destroy_oid_map on the "bitmaps" map, which
    maps commit OIDs to their corresponding bitmaps, the bitmaps
    themselves are not freed. Note here that we recycle already-freed
    ewah_bitmaps into a pool, but these are handled correctly by
    ewah_pool_free().

  - We never bother to free the extended index's "positions" map, which
    we always allocate in load_bitmap().

Fix both of these.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>