t0018: two small fixes

t0018: two small fixes

Even though the three tests that were recently added started their
here-doc with "<<-\EOF", it did not take advantage of that and
instead wrote the here-doc payload abut to the left edge.  Use a tabs
to indent these lines.

More importantly, because these all hardcode the expected output,
which contains the current branch name, they break the CI job that
uses 'main' as the default branch name.

Use

    GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=trunk
    export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME

between the test_description line and ". ./test-lib.sh" line to
force the initial branch name to 'trunk' and expect it to show in
the output.

Signed-off-by: Junio C Hamano <gitster@pobox.com>