t0613: stop setting default initial branch

t0613: stop setting default initial branch

As the tests are all run in separate repositories, set the branch
name to "master" when creating the repository for the tests where
the result depends on the branch name. In order to make it easier to
change the branch name in the future a helper function is used. This
reduces the number of tests that depend on the default branch name
being "master" and removes the last instance of a test file using
"GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master".

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>