internal/lsp: enable go-diff by default

Also, fix a minor staticcheck warning.

Change-Id: Ic500215dac7985a9d8a7189aea4e1be7b20f7780
Reviewed-on: https://go-review.googlesource.com/c/tools/+/204560
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/lsp/cache/check.go

@@ -117,10 +117,7 @@ func (s *snapshot) buildKey(ctx context.Context, id packageID, mode source.Parse
 	}
 
 	// Make sure all of the depList are sorted.
-	var depList []packageID
-	for _, id := range m.deps {
-		depList = append(depList, id)
-	}
+	depList := append([]packageID{}, m.deps...)
 	sort.Slice(depList, func(i, j int) bool {
 		return depList[i] < depList[j]
 	})

internal/lsp/source/options.go

@@ -68,6 +68,7 @@ var (
 		},
 		ComputeEdits: myers.ComputeEdits,
 		Analyzers:    defaultAnalyzers,
+		GoDiff:       true,
 	}
 )