patch-ids: turn off rename detection

patch-ids: turn off rename detection

The patch-id code may be running inside another porcelain
like "git log" or "git format-patch", and therefore may have
set diff_detect_rename_default, either via the diff-ui
config, or by default since 5404c11 (diff: activate
diff.renames by default, 2016-02-25). This is the case even
if a command is run with `--no-renames`, as that is applied
only to the diff-options used by the command itself.

Rename detection doesn't help the patch-id results. It
_may_ actually hurt, as minor differences in the files that
would be overlooked by patch-id's canonicalization might
result in different renames (though I'd doubt that it ever
comes up in practice).

But mostly it is just a waste of CPU to compute these
renames.

Note that this does have one user-visible impact: the
prerequisite patches listed by "format-patch --base". There
may be some confusion between different versions of git as
older ones will enable renames, but newer ones will not.
However, this was already a problem, as people with
different settings for the "diff.renames" config would get
different results. After this patch, everyone should get the
same results, regardless of their config.

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