diff: clean up external-diff argv setup

diff: clean up external-diff argv setup

Since the previous commit, setting up the tempfile for an external diff
uses df->path from the diff_filespec, rather than the logical name. This
means add_external_diff_name() does not need to take a "name" parameter
at all, and we can drop it. And that in turn lets us simplify the
conditional for handling renames (when the "other" name is non-NULL).

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