t4015: let the test pass with any default branch name

t4015: let the test pass with any default branch name

We do not need to hard-code the actual branch name, as we can use the
`test_commit` function to simplify the code and use the tag it
generates, thereby being a lot more precise in what we want.

Strangely enough, this test case would have succeeded even with an
overridden default branch name, obviously for the wrong reason. Let's
verify that it passes for the expected reason, by looking for a
tell-tale in Git's output.

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