git-gui: Refactor mouse clicking on file names/icons.

git-gui: Refactor mouse clicking on file names/icons.

I'm not a huge fan of putting the left and right mouse actions into
the same procedure.  Originally this is how Paul had implemented the
logic in gitool and I had carried some of that over into git-gui, but
now that I'm getting ready to implement right mouse click features to
act on files I really should split this apart.

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