format-patch supports the format.headers configuration for adding
arbitrary email headers to the patches it outputs.  This patch adds
support for an --add-header argument which makes the same feature
available from the command line.  This is useful when the content of
custom email headers must change from branch to branch.

This patch has been sponsored by Grant Street Group
Signed-off-by: NMichael Hendricks <michael@ndrix.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>