t7610: don't assume the checked-out commit

t7610: don't assume the checked-out commit

Always check out the required commit at the beginning of the test so
that a failure in a previous test does not cause the test to work off
of the wrong commit.

This is a step toward making the tests more independent so that if one
test fails it doesn't cause subsequent tests to fail.

Signed-off-by: Richard Hansen <hansenr@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>