t4150 (am): futureproof against failing tests

t4150 (am): futureproof against failing tests

Most tests in t4150 begin by navigating to a sane state and
applying some patch:

	git checkout first &&
	git am patch1

If a previous test left behind unmerged files or a .git/rebase-apply
directory, they are untouched and the test fails, causing later tests
to fail, too.  This is not a problem in practice because none of the
tests leave a mess behind.

But as a futureproofing measure, it is still best to avoid the problem
and clean up at the start of each test.  In particular, this
simplifies the process of adding new tests that are known to fail.

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