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 <spearce@spearce.org>

lib/checkout_op.tcl

@@ -208,7 +208,7 @@ method _update_ref {} {
 				}
 			}
 			default {
-				_error $this "Only 'ff' and 'reset' merge is currently supported."
+				_error $this "Merge strategy '$merge_type' not supported."
 				return 0
 			}
 			}