fetch: split out tests for output format

fetch: split out tests for output format

We're about to introduce a new porcelain mode for the output of
git-fetch(1). As part of that we'll be introducing a set of new tests
that only relate to the output of this command.

Split out tests that exercise the output format of git-fetch(1) so that
it becomes easier to verify this functionality as a standalone unit. As
the tests assume that the default branch is called "main" we set up the
corresponding GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME environment variable
accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>