When searching for keyboard shortcuts in Microsoft Visual Studio 2013 I came across the keyboard shortcut for bringing up the pending changes window:
Press CTRL + 0, P
Now the first time it will bring up the pending changes window, but the comments textbox will not be focused.
Put your cursor in the textbox, write some comments.
Press ALT + I
This will check in your changes.
Now when you make a change to a file and press CTRL + 0, P the comment textbox will be focused and you can directly start typing your comments.