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 <email@example.com>