format-patch: make newline after signature conditional

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 <redacted>
Signed-off-by: Jeremiah Mahler <redacted>
Signed-off-by: Junio C Hamano <redacted>