Merge branch 'jc/countermand-format-attach'

The format.attach configuration variable lacked a way to override a value defined in a lower-priority configuration file (e.g. the system one) by redefining it in a higher-priority configuration file. Now, setting format.attach to an empty string means show the patch inline in the e-mail message, without using MIME attachment. This is a backward incompatible change. * jc/countermand-format-attach: format.attach: allow empty value to disable multi-part messages