t5510: start tracking-ref tests from a known state

t5510: start tracking-ref tests from a known state

We have three sequential tests for for whether tracking refs
are updated by various fetches and pulls; the first two
should not update the ref, and the third should. Each test
depends on the state left by the test before.

This is fragile (a failing early test will confuse later
tests), and means we cannot add more "should update" tests
after the third one.

Let's instead save the initial state before these tests, and
then reset to a known state before running each test.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>