Add a NameRevCommand for describing IDs in terms of refnames

Add a NameRevCommand for describing IDs in terms of refnames

The walk logic does not use RevWalk because it needs to walk all paths
to each of the requested commits, keeping track of each path along which
the commit was found in the RevCommit subclass. From these paths, a
single "best" path is chosen based on the total path length, with a
penalty applied for paths that traverse merges.

This functionality parallels "git name-rev".

Change-Id: I92bfb47dd16c898313d2ee525395609c3bf72ebe