test-lib: do not show advice about init.defaultBranch under --verbose

test-lib: do not show advice about init.defaultBranch under --verbose

Arrange for the advice about naming the initial branch not to be shown
in the --verbose output of the test suite.

Since 675704c74dd (init: provide useful advice about
init.defaultBranch, 2020-12-11) some tests have been very chatty with
repeated occurrences of this multi-line advice. Having it be this
verbose isn't helpful for anyone in the context of git's own test
suite, and it makes debugging tests that use their own "git init"
invocations needlessly distracting.

By setting the GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME variable early in
test-lib.sh itself we'll squash the warning not only for
test_create_repo(), as 675704c74dd explicitly intended, but also for
other "git init" invocations.

And once we'd like to have this configuration set for all "git init"
invocations in the test suite we can get rid of the init.defaultBranch
configuration setting in test_create_repo(), as
repo_default_branch_name() in refs.c will take the GIT_TEST_* variable
over it being set.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>