internal/lsp/cmd: use 'go-diff' by default in format command

Change-Id: Idf6c828721921df21de3d25930580ad5ed5808d3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/202620
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
diff --git a/internal/lsp/cmd/cmd.go b/internal/lsp/cmd/cmd.go
index a87234b..60b307d 100644
--- a/internal/lsp/cmd/cmd.go
+++ b/internal/lsp/cmd/cmd.go
@@ -304,7 +304,8 @@
 			env[l[0]] = l[1]
 		}
 		results[i] = map[string]interface{}{
-			"env": env,
+			"env":     env,
+			"go-diff": true,
 		}
 	}
 	return results, nil