write_sha1_file(): do not use a separate sha1[] array

write_sha1_file(): do not use a separate sha1[] array

In the beginning, write_sha1_file() did not have a way to tell the
caller the name of the object it wrote to the caller.  This was
changed in d6d3f9d0 (This implements the new "recursive tree"
write-tree., 2005-04-09) by adding the "returnsha1" parameter to the
function so that the callers who are interested in the value can
optionally pass a pointer to receive it.

It turns out that all callers do want to know the name of the object
it just has written.  Nobody passes a NULL to this parameter, hence
it is not necessary to use a separate sha1[] array to receive the
result from  write_sha1_file_prepare(), and copy the result to the
returnsha1 supplied by the caller.

Signed-off-by: Junio C Hamano <gitster@pobox.com>