rebase: protect against diff.renames configuration

rebase: protect against diff.renames configuration

We currently do not disable diff.renames configuration while rebase
internally runs "format-patch" to feed "am -3".

The end user configuration for "diff" should not affect the result
produced by the higher level command that is related to "diff" only
because internally it is implemented in terms of it.

For that matter, I have a feeling that format-patch should not even look
at diff.renames, but we seem to have been doing this for a long time so
there is no easy way to fix this thinko.

In any case, here is a much straightforward fix for "rebase".

[jn: with test case from David]

Reported-by: David D. Kilzer <ddkilzer@kilzer.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>