How can I view the original (or "master") version of a photo in Photos without reverting all edits?
While in Edit mode, hold down the Control and M keys.

This will show you the original version of your photo, with no edits applied. If you want to temporarily undo the effect of the color adjustments but keep the cropping and rotation the same, hold down M by itself.

Unlike Aperture:


It doesn’t work in “view mode”; you have to be in edit mode. (To get into edit mode via the keyboard, press Return.)
It’s a hold, not a toggle, so you only see the original photo for as long as you have the keys held down.