Add line diff decorations to editor gutter

Added, modified, and deleted lines will now highlighted
in the gutter for files already checked in to the repository.

src/packages/git-diff/spec/git-diff-spec.coffee

@@ -0,0 +1,63 @@
+RootView = require 'root-view'
+_ = require 'underscore'
+
+describe "GitDiff package", ->
+  editor = null
+
+  beforeEach ->
+    config.set('git-diff.enabled', true)
+    window.rootView = new RootView
+    rootView.attachToDom()
+    rootView.open('sample.js')
+    atom.activatePackage('git-diff')
+    editor = rootView.getActiveView()
+
+  describe "when the editor has modified lines", ->
+    it "highlights the modified lines", ->
+      expect(editor.find('.git-line-modified').length).toBe 0
+      editor.insertText('a')
+      advanceClock(editor.getBuffer().stoppedChangingDelay)
+      expect(editor.find('.git-line-modified').length).toBe 1
+      expect(editor.find('.git-line-modified').attr('lineNumber')).toBe '0'
+
+  describe "when the editor has added lines", ->
+    it "highlights the added lines", ->
+      expect(editor.find('.git-line-added').length).toBe 0
+      editor.moveCursorToEndOfLine()
+      editor.insertNewline()
+      editor.insertText('a')
+      advanceClock(editor.getBuffer().stoppedChangingDelay)
+      expect(editor.find('.git-line-added').length).toBe 1
+      expect(editor.find('.git-line-added').attr('lineNumber')).toBe '1'
+
+  describe "when the editor has removed lines", ->
+    it "highlights the line preceeding the deleted lines", ->
+      expect(editor.find('.git-line-added').length).toBe 0
+      editor.setCursorBufferPosition([5])
+      editor.deleteLine()
+      advanceClock(editor.getBuffer().stoppedChangingDelay)
+      expect(editor.find('.git-line-removed').length).toBe 1
+      expect(editor.find('.git-line-removed').attr('lineNumber')).toBe '4'
+
+  describe "when a modified line is restored to the HEAD version contents", ->
+    it "removes the diff highlight", ->
+      expect(editor.find('.git-line-modified').length).toBe 0
+      editor.insertText('a')
+      advanceClock(editor.getBuffer().stoppedChangingDelay)
+      expect(editor.find('.git-line-modified').length).toBe 1
+      editor.backspace()
+      advanceClock(editor.getBuffer().stoppedChangingDelay)
+      expect(editor.find('.git-line-modified').length).toBe 0
+
+  describe "when a modified file is opened", ->
+    it "highlights the changed lines", ->
+      path = project.resolve('sample.txt')
+      buffer = project.buildBuffer(path)
+      buffer.setText("Some different text.")
+      rootView.open('sample.txt')
+      nextTick = false
+      _.nextTick -> nextTick = true
+      waitsFor -> nextTick
+      runs ->
+        expect(editor.find('.git-line-modified').length).toBe 1
+        expect(editor.find('.git-line-modified').attr('lineNumber')).toBe '0'