pack-bitmap: handle objects at bitmap position zero

`bitmap_position()` only returns a negative value when an object is not present in the bitmap index. In `find_objects()`, we have added a check (11d45a6e6a) to avoid processing a root whose reachability is already represented by the base bitmap, but accidentally uses `pos > 0`. Consequently, it never performs the membership test for an object at position zero. If that object has an individual reachability bitmap, we unnecessarily OR that bitmap into the base again. Otherwise, we add the object to the not-mapped list, only for the subsequent pass to recognize that it is already present. The latter pass correctly treats all non-negative positions as valid, so this does not change the resulting object set, but an off-by-one edge case. Treat position zero as valid by changing the condition to `pos >= 0`. The existing pseudo-merge traversal test exercises this case. Its position-zero commit is presented through multiple roots. Before this change, each occurrence is counted as a bitmap hit; afterwards, only the first occurrence is counted. Assert the resulting hit count to cover the boundary condition. Also cover the non-pseudo-merge case by passing `HEAD` twice. The first occurrence initializes the base from its stored bitmap, and the second must recognize that position zero is already present. Helped-by: Taylor Blau <ttaylorr@openai.com> Signed-off-by: David Lin <davidlin@stripe.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>