Hello!


In relation to the description of `--used-refspec` in the docs https://git-annex.branchable.com/git-annex-unused/ 
I was wondering why it is possible to add a positive refspec without the full path of the ref e.g.

`git annex unused --used-refspec  +master`

while for exclusions (negative refspec) it seems to be necessary to specify the full path, e.g.

`git annex unused --used-refspec  '+*:-refs/heads/master'`

or globbing:

`git annex unused --used-refspec  '+*:-*master'`

I guess the negative form of 

> Each + without a glob adds the literal value to the set.  For example, "+HEAD^" adds "HEAD^".

is actually (intentionally) missing. 
If so, this might be worth noting in the docs.
