t4012: Actually quote the sed script

The nested quoting is not needed in this cases, thus the previous
version did work just fine. Nevertheless the usage is misleading,
so just achieve nested quoting by using double quotes instead. Lower
the probability of breakage in the future and make the code easier
to read.

NOTE: Just dropping the single quotes around the sed arguments would
      have also been possible.

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4012-diff-binary.sh

@@ -64,7 +64,7 @@ test_expect_success 'apply --numstat understands diff --binary format' '
 # apply needs to be able to skip the binary material correctly
 # in order to report the line number of a corrupt patch.
 test_expect_success 'apply detecting corrupt patch correctly' '
-	 git diff | sed -e 's/-CIT/xCIT/' >broken &&
+	 git diff | sed -e "s/-CIT/xCIT/" >broken &&
 	 if git apply --stat --summary broken 2>detected
 	 then
 		echo unhappy - should have detected an error
@@ -79,7 +79,7 @@ test_expect_success 'apply detecting corrupt patch correctly' '
 '
 
 test_expect_success 'apply detecting corrupt patch correctly' '
-	 git diff --binary | sed -e 's/-CIT/xCIT/' >broken &&
+	 git diff --binary | sed -e "s/-CIT/xCIT/" >broken &&
 	 if git apply --stat --summary broken 2>detected
 	 then
 		echo unhappy - should have detected an error