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