repack: simplify handling of --write-bitmap-index

repack: simplify handling of --write-bitmap-index

We previously needed to pass --no-write-bitmap-index
explicitly to pack-objects to override its reading of
pack.writebitmaps from the config. Now that it no longer
does so, we can assume that bitmaps are off by default, and
only turn them on when necessary. This also lets us avoid a
confusing tri-state flag for write_bitmaps.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>