format.attach: allow empty value to disable multi-part messages

format.attach: allow empty value to disable multi-part messages

When a lower precedence configuration file (e.g. /etc/gitconfig)
defines format.attach in any way, there was no way to disable it in
a more specific configuration file (e.g. $HOME/.gitconfig).

Change the behaviour of setting it to an empty string.  It used to
mean that the result is still a multipart message with only dashes
used as a multi-part separator, but now it resets the setting to
the default (which would be to give an inline patch, unless other
command line options are in effect).

Signed-off-by: Junio C Hamano <gitster@pobox.com>