t4150 (am): style fix

Place setup commands in test_expect_success blocks.  This makes the
rare event of the setup commands breaking on some platform easier to
diagnose, and more importantly, it visually distinguishes where
each test begins and ends.

Instead of running test -z against the result of "git diff" command
substitution, use "git diff --exit-code", to improve output when
running with the "-v" option.

Use test_cmp in place of "test $(foo) = $(bar)" for similar reasons.

Remove whitespace after the > and < redirection operators for
consistency with other tests.

The order of arguments to test_cmp is "test_cmp expected actual".

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