t3427: fix erroneous assumption

t3427: fix erroneous assumption

Apart from the `setup` test case, `t3427-rebase-subtree.sh` is made up
exclusively of demonstrations of breakages. The tricky thing about such
demonstrations is that they are often buggy themselves.

In this instance, somewhere over the course of the six iterations
of the patch that eventually made it into Git's `master` as 5f35900849e
(contrib/subtree: Add a test for subtree rebase that loses commits,
2016-06-28), the commit message "files_subtree/master4" was changed to
just "master4", but the test cases still expected the old commit
message.

Let's fix this, at long last.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>