magit/magit — pull request #697 by cwebber (2013-06-21 16:36:52+00:00)
Title: Allow for diffing with a common ancestor if the prefix key is used with ...

...magit-diff

This makes diffs a lot less noisy during code review. For example, if upstream master has added commits that your new branch hasn't yet, you can still diff against master and it'll show you only the differences since the last time you merged.