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>

Documentation/config/format.txt

@@ -3,7 +3,8 @@ format.attach::
 	'format-patch'.  The value can also be a double quoted string
 	which will enable attachments as the default and set the
 	value as the boundary.  See the --attach option in
-	linkgit:git-format-patch[1].
+	linkgit:git-format-patch[1].  To countermand an earlier
+	value, set it to an empty string.
 
 format.from::
 	Provides the default value for the `--from` option to format-patch.