unpack-trees: use repository from index instead of global

unpack-trees: use repository from index instead of global

unpack_trees() currently initializes its repository from the
global 'the_repository', even though a repository instance is
already available via the source index.

Use 'o->src_index->repo' instead of the global variable,
reducing reliance on global repository state.

This is a step towards eliminating global repository usage in
unpack_trees().

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Jayesh Daga <jayeshdaga99@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>