magit-branch-delete: Handle non-unique ref patterns

`magit-branch-delete` treated the first line of `git show-ref` as the branch to be deleted, but this is not always the case. For example, if the current branch is 'some-branch' and there exists another branch 'a/some-branch', trying to delete 'a/some-branch' will result in `magit-branch-delete` saying 'a/some-branch' is checked out. This is because 'a/some-branch' appears first in the 'git show-ref' output, so
``` 
(magit-ref-fullname (magit-get-current-branch))
```   
returns 'refs/heads/a/some-branch' instead of 'refs/heads/some-branch'.
    
This is fixed by using branch names (instead of the full reference name) to test if the branch for deletion is checked out.