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