diff: drop "name" parameter from prepare_temp_file()

diff: drop "name" parameter from prepare_temp_file()

The prepare_temp_file() function takes a diff_filespec as well as a
filename. But it is almost certainly an error to pass in a name that
isn't the filespec's "path" parameter, since that is the only thing that
reliably tells us how to find the content (and indeed, this was the
source of a recently-fixed bug).

So let's drop the redundant "name" parameter and just use one->path
throughout the function. This simplifies the interface a little bit, and
makes it impossible for calling code to get it wrong.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>