internal/lsp/cache: don't construct a new metadata graph if no changes

Change-Id: I3f074d1fd29cf7ad0323cec76154f9b2e31f7356
Reviewed-on: https://go-review.googlesource.com/c/tools/+/415494
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

internal/lsp/cache/graph.go

@@ -32,6 +32,10 @@ type metadataGraph struct {
 // Clone creates a new metadataGraph, applying the given updates to the
 // receiver.
 func (g *metadataGraph) Clone(updates map[PackageID]*KnownMetadata) *metadataGraph {
+	if len(updates) == 0 {
+		// Optimization: since the graph is immutable, we can return the receiver.
+		return g
+	}
 	result := &metadataGraph{metadata: make(map[PackageID]*KnownMetadata, len(g.metadata))}
 	// Copy metadata.
 	for id, m := range g.metadata {