git-gui: Simplify error case for unsupported merge types
If we are given a merge type we don't understand in checkout_op there is probably a bug in git-gui somewhere that allowed this unknown merge strategy to come into this part of the code path. We currently only recognize three merge types ('none', 'ff' and 'reset') but are going to be supporting more in the future. Rather than keep editing this message I'm going with a very generic "Uh, we don't do that!" type of error.
Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>