Add ref completion to git-missing.

I find useful to get the ref completion in the git-missing command,
the behavior is the *like* the one on 'git log --pretty' it use the
'git for-each-ref' command to provide shortname of :
* All local branches.
* All remote/branches.
* All tags.
* The stash.

The output can be a bit huge, but provide a standard git / bash completion.