pack-bitmap.c: teach `bitmap_for_commit()` about incremental MIDXs

pack-bitmap.c: teach `bitmap_for_commit()` about incremental MIDXs

The pack-bitmap machinery uses `bitmap_for_commit()` to locate the
EWAH-compressed bitmap corresponding to some given commit object.

Teach this function about incremental MIDX bitmaps by teaching it to
recur on earlier bitmap layers when it fails to find a given commit in
the current layer.

The changes to do so are as follows:

  - Avoid initializing hash_pos at its declaration, since
    bitmap_for_commit() is now a recursive function and may receive a
    NULL bitmap_index pointer as its first argument.

  - In cases where we would previously return NULL (to indicate that a
    lookup failed and the given bitmap_index does not contain an entry
    corresponding to the given commit), recursively call the function on
    the previous bitmap layer.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>