git-gui: Display commit/tag/remote info in tooltip of revision picker

git-gui: Display commit/tag/remote info in tooltip of revision picker

Our revision chooser mega-widget now sets up tooltips for itself so
that it displays details about a commit (or a tag and the commit
it refers to) when the user mouses over that line in the filtered
ref list.  If the item is from a remote tracking branch then we also
show the remote url and what branch on that remote we fetch from, so
the user has a clear concept of where that revision data originated.

To help the merge dialog I've also added a new constructor that
makes the dialog only offer unmerged revisions (those not in HEAD),
as this allows users to avoid performing merges only to get "Already
up to date" messages back from core Git.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>