t7419: actually test the branch switching

t7419: actually test the branch switching

The submodule repo the test set up had the 'topic' branch checked out,
meaning the repo's default branch (HEAD) is the 'topic' branch.

The following tests then pretended to switch between the default branch
and the 'topic' branch. This was papered over by continually adding
commits to the 'topic' branch and checking if the submodule gets updated
to this new commit.

Return the submodule repo to the 'main' branch after setup so we can
actually test the switching behavior.

Signed-off-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>