Add FILES env variable

[Simx303]

FILES environment variable will be available for scripts by adding $FILES to their script.
It will list all staged files for commit.

Here is an example:

STAGED
---
coolapp/
├─ .editorconfig +
├─ README.md
├─ src/
│  ├─ main.js +
│  ├─ utils.js +


> echo $FILES
.editorconfig src/main.js src/utils.js

> npx do-something $FILES
...

echo "$FILES" | xargs git add at the end will add again the changed files for commit.