format-patch: page output with --stdout

format-patch: page output with --stdout

Pass output through the pager if format-patch is run with --stdout. This
saves the user the trouble of running git with '-p' or piping through a
pager.

setup_pager() already checks if stdout is a tty, so we don't have to
worry about behaviour if the user redirects/pipes stdout. Paging can
also be disabled with the config

  [pager]
      format-patch = false

Add tests to check for these behaviour.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>