Buffer.save only writes to disk if the buffer is modified

This prevents autosave from writing unmodified buffers to disk, which
was causing the tree view to rebuild directory contents based on the
write event just when switching tabs.

src/app/buffer.coffee

@@ -238,7 +238,7 @@ class Buffer
     @undoManager.redo(editSession)
 
   save: ->
-    @saveAs(@getPath())
+    @saveAs(@getPath()) if @isModified()
 
   saveAs: (path) ->
     unless path then throw new Error("Can't save buffer with no file path")