tests: fix non-portable "${var:-"str"}" construct

On both AIX 7200-00-01-1543 and FreeBSD 11.2-RELEASE-p2 the
"${var:-"str"}" syntax means something different than what it does
under the bash or dash shells.

Both will consider the start of the new unescaped quotes to be a new
argument to test_expect_success, resulting in the following error:

    error: bug in the test script: 'git diff-tree initial # magic
    is (not' does not look like a prereq

Fix this by removing the redundant quotes. There's no need for them,
and the resulting code works under all the aforementioned shells. This
fixes a regression in c2f1d39 ("t4013: test new output from diff
--abbrev --raw", 2017-12-03) first released with Git v2.16.0.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

t/t4013-diff-various.sh

@@ -140,7 +140,7 @@ do
 	expect="$TEST_DIRECTORY/t4013/diff.$test"
 	actual="$pfx-diff.$test"
 
-	test_expect_success "git $cmd # magic is ${magic:-"(not used)"}" '
+	test_expect_success "git $cmd # magic is ${magic:-(not used)}" '
 		{
 			echo "$ git $cmd"
 			case "$magic" in