Allow Diff view during pull request creation

My team has been making heavy use of the pull request feature since it was introduced. One of the constant requests I hear from them is for the ability to use the Diff feature prior to saving the pull request.

The purposes of reviewing the Diff before issuing the pull request:

• the originator can verify that the proper changes are in the request
• the originator can identify any conflicts in the request
• reviewers do not receive 'notification spam' for a pull request that may be invalid/incomplete

Our current workaround for this is:

• create the pull request without assigning reviewers
• review the diff on the newly created pull request
• in case of conflicts, decline the pull request
• otherwise, edit the pull request and add reviewers