memoize Buffer @contentOnDisk so we don't re-read the whole file on every buffer-change event

src/app/buffer.coffee

@@ -14,6 +14,7 @@ class Buffer
   @idCounter = 1
   undoManager: null
   modified: null
+  contentOnDisk: null
   modifiedOnDisk: null
   lines: null
   file: null
@@ -30,7 +31,8 @@ class Buffer
     if path
       throw "Path '#{path}' does not exist" unless fs.exists(path)
       @setPath(path)
-      @setText(fs.read(@getPath()))
+      @contentOnDisk = fs.read(@getPath())
+      @setText(@contentOnDisk)
     else
       @setText('')
 
@@ -68,7 +70,8 @@ class Buffer
       @trigger "path-change", this
 
   reload: ->
-    @setText(fs.read(@file.getPath()))
+    @contentOnDisk = fs.read(@getPath())
+    @setText(@contentOnDisk)
     @modified = false
     @modifiedOnDisk = false
 
@@ -236,6 +239,7 @@ class Buffer
     @modified = false
     @modifiedOnDisk = false
     @setPath(path)
+    @contentOnDisk = @getText()
     @trigger 'after-save'
     @trigger 'buffer-change'
 
@@ -249,7 +253,7 @@ class Buffer
     @modified
 
   contentDifferentOnDisk: ->
-    fs.read(@file.getPath()) != @getText()
+    @contentOnDisk != @getText()
 
   getAnchors: -> new Array(@anchors...)