I liked how magit, the git porcelain that comes with emacs, allowed you to
select what hunks to commit inside each file.
This is helpful if while making changes you make many separate unrelated changes in the same file and want these separate changes to be in separate commits.
Turns out, plain git can do that too, with the git add --patch option. It
interactively goes through each hunk (a part of a file that has been
changed) and asks you if you want it to be staged.