completion: tests: always use 'master' for default initial branch name

completion: tests: always use 'master' for default initial branch name

The default initial branch name can normally be configured using the
GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME environment variable.  However,
when testing e.g. <rev> completion it's convenient to know the
exact initial branch name that will be used.

To achieve that without too much trouble it is considered sufficient
to force the default initial branch name to 'master' for all of
t9902-completion.sh.

Signed-off-by: Britton Leo Kerin <britton.kerin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>