exclude unformattable from interesting refs

`magit-format-ref' gives nil for a ref which is immediately under the
remotes dir, or isn't under any of heads, tags, or remotes.
`magit-list-interesting-refs' should not return the nils, otherwise
`ido-completing-read' throws an error.

Fixes #734.
Merge pull request #831 from npostavs/filter-odd-refs

exclude unformattable from interesting refs