Make :checkout-head-revision command work w/ multiple repos

spec/text-editor-spec.coffee

@@ -4018,3 +4018,25 @@ describe "TextEditor", ->
       editor.setPlaceholderText('OK')
       expect(handler).toHaveBeenCalledWith 'OK'
       expect(editor.getPlaceholderText()).toBe 'OK'
+
+  describe ".checkoutHeadRevision()", ->
+    it "reverts to the version of its file checked into the project repository", ->
+      atom.config.set("editor.confirmCheckoutHeadRevision", false)
+
+      editor.setCursorBufferPosition([0, 0])
+      editor.insertText("---\n")
+      expect(editor.lineTextForBufferRow(0)).toBe "---"
+
+      waitsForPromise ->
+        editor.checkoutHeadRevision()
+
+      runs ->
+        expect(editor.lineTextForBufferRow(0)).toBe "var quicksort = function () {"
+
+    describe "when there's no repository for the editor's file", ->
+      it "doesn't do anything", ->
+        editor = new TextEditor({})
+        editor.setText("stuff")
+        editor.checkoutHeadRevision()
+
+        waitsForPromise -> editor.checkoutHeadRevision()

src/text-editor-element.coffee

@@ -337,7 +337,7 @@ atom.commands.add 'atom-text-editor:not([mini])', stopEventPropagationAndGroupUn
   'editor:newline-below': -> @insertNewlineBelow()
   'editor:newline-above': -> @insertNewlineAbove()
   'editor:toggle-line-comments': -> @toggleLineCommentsInSelection()
-  'editor:checkout-head-revision': -> atom.project.getRepositories()[0]?.checkoutHeadForEditor(this)
+  'editor:checkout-head-revision': -> @checkoutHeadRevision()
   'editor:move-line-up': -> @moveLineUp()
   'editor:move-line-down': -> @moveLineDown()
   'editor:duplicate-lines': -> @duplicateLines()

src/text-editor.coffee

@@ -12,6 +12,7 @@ DisplayBuffer = require './display-buffer'
 Cursor = require './cursor'
 Selection = require './selection'
 TextMateScopeSelector = require('first-mate').ScopeSelector
+{Directory} = require "pathwatcher"
 
 # Public: This class represents all essential editing state for a single
 # {TextBuffer}, including cursor and selection positions, folds, and soft wraps.
@@ -647,6 +648,14 @@ class TextEditor extends Model
     else
       @isModified() and not @buffer.hasMultipleEditors()
 
+  checkoutHeadRevision: ->
+    if filePath = this.getPath()
+      atom.project.repositoryForDirectory(new Directory(path.dirname(filePath)))
+        .then (repository) =>
+          repository?.checkoutHeadForEditor(this)
+    else
+      Promise.resolve(false)
+
   ###
   Section: Reading Text
   ###