t6036, t6042: use test_create_repo to keep tests independent

t6036, t6042: use test_create_repo to keep tests independent

These tests used pretty strong measures to get a clean slate:
        git rm -rf . &&
        git clean -fdqx &&
        rm -rf .git &&
        git init &&
It's easier, safer (what if a previous test has a bug and accidentally
changes into a directory outside the test path?), and allows re-inspecting
test setup later if we instead just use test_create_repo to put different
tests into separate sub-repositories.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>