Add confirmation dialog to checkoutHead

There have been a few reports of files mysteriously going back to the
last version in the tree. My theory is that it is because of people
fat-fingering the Undo command because the default keyboard mapping is
too similar. This will at least prevent most accidental data loss.

src/editor.coffee

@@ -436,8 +436,14 @@ class Editor extends Model
   saveAs: (filePath) -> @buffer.saveAs(filePath)
 
   checkoutHead: ->
-    if filePath = @getPath()
-      atom.project.getRepo()?.checkoutHead(filePath)
+    if (filePath = @getPath()) and (repo = atom.project.getRepo())
+      atom.confirm
+        message: "Are you sure you want to revert this file to the last Git commit?"
+        detailedMessage: "You are reverting: #{filePath}"
+        buttons:
+          "Revert": ->
+            repo.checkoutHead(filePath)
+          "Cancel": null
 
   # Copies the current file path to the native clipboard.
   copyPathToClipboard: ->