fix(watcher): ignore double "add" events

On linux, `fs.watch` (which chokidar uses when `usePolling: false`)
fires "add" events twice. This change makes `fileList` to ignore the
second event.

lib/file-list.js

@@ -271,14 +271,16 @@ var List = function(patterns, excludes, emitter, preprocess, batchInterval) {
       return done();
     }
 
+    var addedFile = new File(path);
+    buckets[i].push(addedFile);
+
     return fs.stat(path, function(err, stat) {
       // in the case someone refresh() the list before stat callback
       if (self.buckets === buckets) {
+        addedFile.mtime = stat.mtime;
 
-        var file = new File(path, stat.mtime);
-        return preprocess(file, function() {
+        return preprocess(addedFile, function() {
           // TODO(vojta): ignore if refresh/reload happens
-          buckets[i].push(file);
           log.info('Added file "%s".', path);
           fireEventAndDefer();
           done();

test/unit/file-list.spec.coffee

@@ -296,6 +296,30 @@ describe 'file-list', ->
           done()
 
 
+    it 'should ignore very quick double "add"', (done) ->
+      # On linux fs.watch (chokidar with usePolling: false) fires "add" event twice.
+      # This checks that we only stat and preprocess the file once.
+
+      sinon.spy mockFs, 'stat'
+      list = new m.List patterns('/a.*'), [], emitter, preprocessMock
+
+      pending = 2
+      finish = ->
+        pending--
+        if pending is 0
+          expect(preprocessMock).to.have.been.calledOnce
+          expect(mockFs.stat).to.have.been.calledOnce
+          done()
+
+      refreshListAndThen (files) ->
+        preprocessMock.reset()
+        mockFs.stat.reset()
+
+        list.addFile '/a.js', finish
+        # fire again, before the stat gets back
+        list.addFile '/a.js', finish
+
+
     it 'should set proper mtime of new file', (done) ->
       list = new m.List patterns('/a.*'), [], emitter, preprocessMock