git-gui: Replace merge dialog with our revision picker widget

git-gui: Replace merge dialog with our revision picker widget

Now that we only support merging one branch we can offer the user
a better user interface experience by allowing them to select the
revision they want to merge through our revision picking widget.

This change neatly solves the problem of locating a branch out of
a sea of 200 tracking branches, and of dealing with very long branch
names that all have a common prefix.

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