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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>