format-patch: make newline after signature conditional

When we print an email signature, we print the divider "-- \n", then the signature string, then two newlines. Usually the signature is a one-liner (and the default is just the git version), so the extra newline makes sense. But one could easily specify a multi-line signature, like this: git format-patch --signature='this is my long signature it has multiple lines ' ... and it may end with its own newline, in which case we do not have to add yet another one. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>