t1450-fsck: exec-bit is not needed to make loose object writable

A test case wants to append stuff to a loose object file to ensure
that this kind of corruption is detected. To make a read-only loose
object file writable with chmod, it is not necessary to also make
it executable. Replace the bitmask 755 with the instruction +w to
request only the write bit and to also heed the umask. And get rid
of a POSIXPERM prerequisite, which is unnecessary for the test.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1450-fsck.sh

@@ -94,13 +94,13 @@ test_expect_success 'object with hash and type mismatch' '
 	)
 '
 
-test_expect_success POSIXPERM 'zlib corrupt loose object output ' '
+test_expect_success 'zlib corrupt loose object output ' '
 	git init --bare corrupt-loose-output &&
 	(
 		cd corrupt-loose-output &&
 		oid=$(git hash-object -w --stdin --literally </dev/null) &&
 		oidf=objects/$(test_oid_to_path "$oid") &&
-		chmod 755 $oidf &&
+		chmod +w $oidf &&
 		echo extra garbage >>$oidf &&
 
 		cat >expect.error <<-EOF &&