format-patch: ensure that --rfc and -k are mutually exclusive
Fix a bug that allows the "--rfc" and "-k" options to be specified together
when "git format-patch" is executed, which was introduced in the commit
e0d7db7423a9 ("format-patch: --rfc honors what --subject-prefix sets").
Add a couple of additional tests to t4014, to cover additional cases of
the mutual exclusivity between different "git format-patch" options.
Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>