submodule: use submodule repository when preparing summary

submodule: use submodule repository when preparing summary

In show_submodule_header(), we gather the left and right commits
of the submodule repository, as well as the merge bases. However,
prepare_submodule_summary() initializes the rev_info with the_repository,
so we end up parsing the commit in the wrong repository.

This results in a fatal error in parse_commit_in_graph(), since the
passed item does not belong to the repository's commit graph.

Signed-off-by: Michael Forney <redacted>
Acked-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>