Uses git-diff-index.

Also adds `git ec` alias so maybe I remember to use this.

Use fzf to select files from changed files (including untracked) in a
working tree.  Thanks to @tomasino@mastodon.sdf.org for the git subcommand
idea here - these are scripts rather than aliases, but the interface
is pretty well the same:

https://mastodon.social/@brennen/101440377610036479