Fix ID comparison in osmium merge to match osmium sort behavior

[zihaooo]

Description

When using osmium sort, object IDs are sorted according to the following rule:

Compare two object IDs. Order is as follows: 0 first, then negative IDs, then positive IDs — both ordered by their absolute values.
(See object_comparisons.hpp#L83-L110)

However, in osmium merge, the order validation currently relies on a plain numerical comparison of IDs.
As a result, osmium merge may fail when merging files produced by osmium sort if negative IDs are present.

Proposed fix

This pull request addresses the mismatch by reusing the id_order comparator defined in
object_comparisons.hpp.
Specifically, it replaces the simple numeric comparison in osmium merge with id_order, ensuring consistency with osmium sort.