directory-signature parsing is weirdly ambiguous

The syntax is given as:

 * **`directory-signature`** \[_Algorithm_\] _identity_ _signing-key-digest_ ..

We're told that lines with an unknown Algorithm should be ignored.

Optional items at the beginning are obviously a problem. In this case the nominal parsing seems want to depend on whether the first word looks like hex or not?

This is very strange. See also #322.