[Feature request] Narrowing search/autocomplete for branch selection (and other places)

[rzikm]

Currently, user has to write the exact branch name during checkout (or at least I didn't notice any auto-complete/suggestion functionality). Magit uses narrowing search packages like Helm to simplify selecting branch during checkout-branch and other actions. It would be nice to replicate at least some subset of the narrowing search framework in gitu.