rebase-am: explicitly disable cover-letter

If the user has a cover-letter configuration set to anything other
than 'false', 'git format-patch' may generate a cover letter, which
has no place in "format-patch | am" pipeline.

The internal invocation of format-patch must explicitly override the
configuration from the command line, just like --src-prefix and other
options already do.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>