Merge branch 'dd/format-patch-rfc-updates'

"git format-patch --rfc --subject-prefix=<foo>" used to ignore the "--subject-prefix" option and used "[RFC PATCH]"; now we will add "RFC" prefix to whatever subject prefix is specified. This is a backward compatible change that may deserve a note. * dd/format-patch-rfc-updates: format-patch: --rfc honors what --subject-prefix sets