git-gui: Include commit id/subject in merge choices

git-gui: Include commit id/subject in merge choices

When merging branches using our local merge feature it can be
handy to know the first few digits of the commit the ref points
at as well as the short description of the branch name.

Unfortunately I'm unable to use three listboxes in a row, as Tcl
freaks out and refuses to let me have a selection in more than
one of them at any given point in time.  So instead we use a
fixed width font in the existing listbox and organize the data
into three columns.  Not nearly as nice looking, but users can
continue to use the listbox's features.

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