Change sed i\ usage to something Solaris' sed can handle

Solaris' sed was choking on the i\ commands used in
t4015-diff-whitespace as it couldn't parse the program properly.
Modify two uses of sed that worked in GNU sed but not Solaris'
(/usr/bin or /usr/xpg4/bin) to an equivalent form that is handled
properly by both.

Signed-off-by: Ben Walton <bdwalton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -145,7 +145,8 @@ test_expect_success 'another test, with --ignore-space-at-eol' 'test_cmp expect
 test_expect_success 'ignore-blank-lines: only new lines' '
 	test_seq 5 >x &&
 	git update-index x &&
-	test_seq 5 | sed "/3/i \\
+	test_seq 5 | sed "/3/i\\
+\
 " >x &&
 	git diff --ignore-blank-lines >out &&
 	>expect &&
@@ -155,7 +156,8 @@ test_expect_success 'ignore-blank-lines: only new lines' '
 test_expect_success 'ignore-blank-lines: only new lines with space' '
 	test_seq 5 >x &&
 	git update-index x &&
-	test_seq 5 | sed "/3/i \ " >x &&
+	test_seq 5 | sed "/3/i\\
+ " >x &&
 	git diff -w --ignore-blank-lines >out &&
 	>expect &&
 	test_cmp out expect