Re: [PATCH 05/24] pack-bitmap: move some initialization to `bitmap_writer_init()`

> The pack-bitmap-writer machinery uses a oidmap (backed by khash.h) to
> map from commits selected for bitmaps (by OID) to a bitmapped_commit
> structure (containing the bitmap itself, among other things like its XOR
> offset, etc.)
> 
> This map was initialized at the end of `bitmap_writer_build()`. New
> entries are added in `pack-bitmap-write.c::store_selected()`, which is
> called by the bitmap_builder machinery (which is responsible for
> traversing history and generating the actual bitmaps).
> 
> Reorganize when this field is initialized and when entries are added to
> it so that we can quickly determine whether a commit is a candidate for
> pseudo-merge selection, or not (since it was already selected to receive
> a bitmap, and thus is ineligible for pseudo-merge inclusion).