builtin/rev-list: fix leaking bitmap index when calculating disk usage

builtin/rev-list: fix leaking bitmap index when calculating disk usage

git-rev-list(1) can speed up its object size calculations for reachable
objects via a bitmap walk, if there is any bitmap. This is done in
`try_bitmap_disk_usage()`, which tries to optimistically load the bitmap
and then use it, if available. It never frees it though, leading to a
memory leak. Fix this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>