store hash in depndencyTemplates Map instead of comparing identify

This restores original performance on incremental builds

fixes #5289
fixes #5277

lib/Compilation.js

@@ -80,6 +80,7 @@ class Compilation extends Tapable {
 		this.children = [];
 		this.dependencyFactories = new Map();
 		this.dependencyTemplates = new Map();
+		this.dependencyTemplates.set("hash", "");
 		this.childrenCounters = {};
 	}
 

lib/NormalModule.js

@@ -54,6 +54,8 @@ class NonErrorEmittedError extends WebpackError {
 	}
 }
 
+const dependencyTemplatesHashMap = new WeakMap();
+
 class NormalModule extends Module {
 
 	constructor(request, userRequest, rawRequest, loaders, resource, parser) {
@@ -74,7 +76,6 @@ class NormalModule extends Module {
 		this.assets = {};
 		this.built = false;
 		this._cachedSource = null;
-		this._dependencyTemplatesHashMap = new Map();
 	}
 
 	identifier() {
@@ -307,12 +308,10 @@ class NormalModule extends Module {
 	}
 
 	getHashDigest(dependencyTemplates) {
-		let dtId = this._dependencyTemplatesHashMap.get(dependencyTemplates);
-		if(dtId === undefined)
-			this._dependencyTemplatesHashMap.set(dependencyTemplates, dtId = this._dependencyTemplatesHashMap.size + 1);
+		let dtHash = dependencyTemplatesHashMap.get("hash");
 		const hash = crypto.createHash("md5");
 		this.updateHash(hash);
-		hash.update(`${dtId}`);
+		hash.update(`${dtHash}`);
 		return hash.digest("hex");
 	}
 

lib/optimize/ConcatenatedModule.js

@@ -301,6 +301,7 @@ class ConcatenatedModule extends Module {
 			this.rootModule,
 			moduleToInfoMap
 		));
+		innerDependencyTemplates.set("hash", innerDependencyTemplates.get("hash") + this.rootModule.identifier());
 
 		// Generate source code and analyse scopes
 		// Prepare a ReplaceSource for the final source