Manage unknown refs

Only refs/heads and refs/remotes have meaning for a default git repo.

• Allow to configure filtering for unknown refs
• Allow to customize unknown refs (adding meaningful references to a list like refs/bisect)
• Group references under a revealer (for easy access)
• Enable Search on sidebar (with a shortcut or similar, to find reference in long list
• Special render to avoid just sha representation of unknown refs (like bisect good, bad...)

fixes #140 (closed)