use birthtime for folders instead of modified time

lib/DirectoryWatcher.js

@@ -236,12 +236,12 @@ class DirectoryWatcher extends EventEmitter {
 		});
 	}
 
-	setDirectory(directoryPath, mtime, initial, type) {
+	setDirectory(directoryPath, birthtime, initial, type) {
 		if (this.checkIgnore(directoryPath)) return;
 		if (directoryPath === this.path) {
 			if (!initial) {
 				this.forEachWatcher(this.path, w =>
-					w.emit("change", directoryPath, mtime, type, initial)
+					w.emit("change", directoryPath, birthtime, type, initial)
 				);
 			}
 		} else {
@@ -258,14 +258,14 @@ class DirectoryWatcher extends EventEmitter {
 
 				let safeTime;
 				if (initial) {
-					safeTime = Math.min(now, mtime) + FS_ACCURACY;
+					safeTime = Math.min(now, birthtime) + FS_ACCURACY;
 				} else {
 					safeTime = now;
 				}
 
 				this.forEachWatcher(directoryPath, w => {
 					if (!initial || w.checkStartTime(safeTime, false)) {
-						w.emit("change", mtime, type);
+						w.emit("change", birthtime, type);
 					}
 				});
 				this.forEachWatcher(this.path, w => {
@@ -437,7 +437,7 @@ class DirectoryWatcher extends EventEmitter {
 					} else if (stats.isDirectory()) {
 						this.setDirectory(
 							filePath,
-							+stats.mtime || +stats.ctime || 1,
+							+stats.birthtime || 1,
 							false,
 							eventType
 						);
@@ -669,7 +669,7 @@ class DirectoryWatcher extends EventEmitter {
 							if (!initial || !this.directories.has(itemPath))
 								this.setDirectory(
 									itemPath,
-									+stats.mtime || +stats.ctime || 1,
+									+stats.birthtime || 1,
 									initial,
 									"scan (dir)"
 								);

test/Watchpack.js

@@ -885,6 +885,32 @@ describe("Watchpack", function() {
 		});
 	});
 
+	it("should not report changes to a folder watched as file when items are added", function(done) {
+		var w = new Watchpack({
+			aggregateTimeout: 100
+		});
+		w.on("aggregated", () => {
+			done(new Error("should not fire"));
+		});
+		testHelper.dir("dir");
+		testHelper.file("dir/a");
+		testHelper.tick(1000, () => {
+			testHelper.file("dir/b");
+			w.watch({
+				files: [path.join(fixtures, "dir")],
+				startTime: Date.now()
+			});
+			testHelper.tick(1000, () => {
+				testHelper.file("dir/c");
+				testHelper.tick(1000, () => {
+					// no event fired
+					w.close();
+					done();
+				});
+			});
+		});
+	});
+
 	it("should report removal of file and directory if it is missing in initial scan", function(done) {
 		var w = new Watchpack({
 			aggregateTimeout: 1000