format-patch: add "--signature-file=<file>" option

Add an option to format-patch for reading a signature from a file.

  $ git format-patch -1 --signature-file=$HOME/.signature

The config variable `format.signaturefile` can also be used to make
this the default.

  $ git config format.signaturefile $HOME/.signature

  $ git format-patch -1

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>