lint-staged: When a watcher process being used, git stash restoration might fail

I’ve been testing beta-6 and noticed that if there is a watcher process that modifies files is being used, lint-staged will fail ungracefully:

 M src/components/primitives/Text/Text.js
MM src/exercises/3-Primitives/Text.js
 M src/exercises/3-Primitives/final/Text.js
MM src/exercises/3-Primitives/templates/Text.js

component-driven-development on  rerere2 [⇡$!+] is 📦 v0.1.0 via ⬢ v10.15.3 
➜ gc  
husky > pre-commit (node v10.15.3)
  ✔ Preparing...
  ✔ Running tasks...
  ✖ Applying modifications...
    → Unstaged changes could not be restored due to a merge conflict!
    Cleaning up...
Unstaged changes could not be restored due to a merge conflict!
husky > pre-commit hook failed (add --no-verify to bypass)

component-driven-development on  rerere2 [⇡=$+] is 📦 v0.1.0 via ⬢ v10.15.3 took 2s 
➜ g st
## rerere2...origin/rerere2 [ahead 2]
M  src/components/primitives/Text/Text.js
M  src/exercises/3-Primitives/Text.js
UU src/exercises/3-Primitives/final/Text.js
M  src/exercises/3-Primitives/templates/Text.js

Notice how after the fail my index looks very differently having staged files that weren’t staged before the commit attempt. Ultimately, the new index is exactly the changes I didn’t want to commit in the first attempt.

Also the error message isn’t helpful in resolving the situation.

Steps to repro:

1. Checkout https://github.com/component-driven/component-driven-development
2. Run npm run generate:watch
3. Edit src/exercises/3-Primitives/templates/Text.js and see how src/exercises/3-Primitives/final/Text.js and being updated.
4. Add those changes to the index by git add .
5. Try to commit

I realize this is might look like a rare scenario at first, but I believe most of issues with lint-staged staging algorithm came from webpack (or other tools) users running it in watch mode in parallel.