t3401: use test_commit in setup

t3401: use test_commit in setup

Simplify t3401 by using test_commit in the setup. This lets us refer
to commits using their tags and there is no longer a need to create
the branch my-topic-branch-merge. Also, the branch master-merge points
to the same commit as master (even before this change), so that branch
does not need to be created either.

While at it, replace "test ! -d" by "test_path_is_missing".

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>