contrib/subtree: fix tests with reftable backend

contrib/subtree: fix tests with reftable backend

One git-subtree test-case relies on git internals to infer the
default branch name. This test fails with the new reftable
backend.

    GIT_TEST_DEFAULT_REF_FORMAT=reftable \
      meson test t7900-subtree

This test script already sets

    GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main

which eliminates the need to infer a branch name at runtime.
Hardcode the branch name.

Signed-off-by: Colin Stagner <ask+git@howdoi.land>
Signed-off-by: Junio C Hamano <gitster@pobox.com>