Update the buffer being used when the editor path changes

Previously the status indicator would only work for the first
buffer open since only the buffer used when the extension was
initialized was subscribed to for change events.

spec/app/status-bar-spec.coffee

@@ -1,6 +1,7 @@
 $ = require 'jquery'
 RootView = require 'root-view'
 StatusBar = require 'status-bar'
+fs = require 'fs'
 
 describe "StatusBar", ->
   [rootView, editor, statusBar, buffer] = []
@@ -55,6 +56,9 @@ describe "StatusBar", ->
 
   describe "when the buffer content has changed from the content on disk", ->
     it "disables the buffer modified indicator on save", ->
+      path = "/tmp/atom-whitespace.txt"
+      fs.write(path, "")
+      rootView.open(path)
       expect(statusBar.bufferModified.text()).toBe ''
       editor.insertText("\n")
       expect(statusBar.bufferModified.text()).toBe '*'
@@ -75,6 +79,20 @@ describe "StatusBar", ->
       editor.undo()
       expect(statusBar.bufferModified.text()).toBe ''
 
+  describe "when the buffer changes", ->
+    it "updates the buffer modified indicator for the new buffer", ->
+      expect(statusBar.bufferModified.text()).toBe ''
+      rootView.open(require.resolve('fixtures/sample.txt'))
+      editor.insertText("\n")
+      expect(statusBar.bufferModified.text()).toBe '*'
+
+    it "doesn't update the buffer modified indicator for the old buffer", ->
+     oldBuffer = editor.getBuffer()
+     expect(statusBar.bufferModified.text()).toBe ''
+     rootView.open(require.resolve('fixtures/sample.txt'))
+     oldBuffer.setText("new text")
+     expect(statusBar.bufferModified.text()).toBe ''
+
   describe "when the associated editor's cursor position changes", ->
     it "updates the cursor position in the status bar", ->
       editor.setCursorScreenPosition([1, 2])

src/app/status-bar.coffee

@@ -24,18 +24,24 @@ class StatusBar extends View
 
   initialize: (@rootView, @editor) ->
     @updatePathText()
-    @editor.on 'editor-path-change', => @updatePathText()
+    @editor.on 'editor-path-change', =>
+      @subscribeToBuffer()
+      @updatePathText()
 
     @updateCursorPositionText()
     @editor.on 'cursor-move', => @updateCursorPositionText()
 
+    @subscribeToBuffer()
+
+  subscribeToBuffer: ->
+    @buffer?.off '.status-bar'
+    @buffer = @editor.getBuffer()
+    @buffer.on 'change.status-bar', => @updateBufferModifiedText()
+    @buffer.on 'after-save.status-bar', => @updateBufferModifiedText()
     @updateBufferModifiedText()
-    @editor.getBuffer().on 'change', => @updateBufferModifiedText()
-    @editor.getBuffer().on 'after-save', => @updateBufferModifiedText()
 
   updateBufferModifiedText: ->
-    buffer = @editor.getBuffer()
-    if buffer.isModified() and buffer.contentDifferentOnDisk()
+    if @buffer.isModified() and @buffer.contentDifferentOnDisk()
       @bufferModified.text('*')
     else
       @bufferModified.text('')