pack-bitmap: fix inverted binary search in `pseudo_merge_at()`

pack-bitmap: fix inverted binary search in `pseudo_merge_at()`

The binary search in `pseudo_merge_at()` has its "lo" and "hi" updates
swapped: when the midpoint's offset is less than the target, it sets `hi
= mi` (searching left) instead of `lo = mi + 1` (searching right), and
vice versa.

This means that lookups for pseudo-merges whose offset is not near the
midpoint of the pseudo-merge table are likely to fail.

In practice, with a single pseudo-merge group this is masked because the
lone entry is always at the midpoint. With multiple groups, the inverted
comparisons cause lookups to search in the wrong direction, potentially
missing entries.

Swap the "lo" and "hi" assignments to search in the correct direction,
making it possible to apply pseudo-merges during fill-in when more than
one pseudo-merge exists in a group.

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