t1300: avoid relying on a bug

The test case 'unset with cont. lines' relied on a bug that is about to
be fixed: it tests *explicitly* that removing the last entry from a
config section leaves an *empty* section behind.

Let's fix this test case not to rely on that behavior, simply by
preventing the section from becoming empty.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t1300-config.sh

@@ -108,6 +108,7 @@ bar = foo
 [beta]
 baz = multiple \
 lines
+foo = bar
 EOF
 
 test_expect_success 'unset with cont. lines' '
@@ -118,6 +119,7 @@ cat > expect <<\EOF
 [alpha]
 bar = foo
 [beta]
+foo = bar
 EOF
 
 test_expect_success 'unset with cont. lines is correct' 'test_cmp expect .git/config'