t6010 (merge-base): modernize style

t6010 (merge-base): modernize style

Guard setup with test_expect_success, put the opening quote
starting each test on the same line as the test_expect_* invocation,
and combine related actions into single tests.

While at it:

 - use test_cmp instead of expr or test $foo = $bar, for more helpful
   output with -v when tests fail;

 - use test_commit for brevity.

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