Merge branch 'jt/format-patch-rfc'

In some projects, it is common to use "[RFC PATCH]" as the subject prefix for a patch meant for discussion rather than application. A new option "--rfc" was a short-hand for "--subject-prefix=RFC PATCH" to help the participants of such projects. * jt/format-patch-rfc: format-patch: add "--rfc" for the common case of [RFC PATCH]