Merge branch 'jk/gcc15'

GCC 15 compatibility updates.

* jk/gcc15:
  object-file: inline empty tree and blob literals
  object-file: treat cached_object values as const
  object-file: drop oid field from find_cached_object() return value
  object-file: move empty_tree struct into find_cached_object()
  object-file: drop confusing oid initializer of empty_tree struct
  object-file: prefer array-of-bytes initializer for hash literals