cmd/gomtree/main.go: Handle Extra and Missing cases
cmd/gomtree/main.go: Handle Extra and Missing cases

The BSD format needed a slight tweak to handle mtree.Extra
and mtree.Missing cases.  It currently only handled the 'Modified'
cases and therefore was not showing missing or extra files during
validation.

vbatts/go-mtree,vbatts/go-mtree