TIL: ediff-revision

When editing a file in some git branch I often want to take a quick look at the same file in other branch (usually master).  I always did it by

 git diff HEAD master -- file.rb 

but today I discovered there's an emacs way to do it.

 M-x ediff-revision

. it asks you for a file (defaults to the current one), and two branches. And that's it!