t3508 (cherry-pick): futureproof against unmerged files

t3508 (cherry-pick): futureproof against unmerged files

Each of the tests in t3508 begins by navigating to a sane state:

	git checkout master &&
	git reset --hard $commit

If a previous test left unmerged files around, they are untouched and
the checkout fails, causing later tests to fail, too.  This is not a
problem in practice because no test except the final one produces
unmerged files.

But as a futureproofing measure, it is still best to avoid the problem
with 'checkout -f'.  In particular, this is needed for new tests to be
added to the end of the script.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>