It has been enabled by default for over three years now:

commit 5d2a30d7d8777319c745804f040fa405d02169ce
Author:     Junio C Hamano <gitster@pobox.com>
CommitDate: 2016-04-03 10:29:22 -0700

    Merge branch 'mm/diff-renames-default'

https://github.com/git/git/commit/5d2a30d7d8777319c745804f040fa405d02169ceSigned-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>