Run Rubocop in Git's pre-commit hook

pre-commit

```
#!/bin/sh
echo "\nRunning rubocop 🚓 💨 💨 💨\n"

declare -a ERRORS=()

for file in $(git diff --cached --name-only | grep -E '.rb')
do
	ERRORS+=("$(rubocop $file | grep -e 'C:' -e 'E:')")
done

if [[ "$ERRORS" != "" ]]; then
	echo "\n 🚨 BEE-BOP! There are some things that you need to fix before commiting!\n"
	history -p "${ERRORS[@]}"
  exit 1
fi

echo "Done! Rubocop has no complains! ✅\n"

exit 0
```

#####
Two steps to get it working:
1) Run this command in your root directory: `mv .git/hooks/pre-commit.sample .git/hooks/pre-commit && chmod +x .git/hooks/pre-commit`
2) Replace the content of the file with the bash script above.

NOTE:
- The first line tells it what intepreter to use, here it's using bash. You can do #!/bin/sh ruby if you want to write a Ruby script.
- `mv` command is to rename the `.githook` from a sample to a legit one. Then `chmod` makes it executable so that it'll actually run.

@edisonywh thank you so much for this it really helped me. The only thing that I had to add is instead of using
$(git diff --cached --name-only | grep -E '.rb') to use $(git diff --cached --name-only | grep -E '\.rb$') as in my case it was catching .html.erb files which rubocop can't really format for me

@edisonywh thanks for the script!

we ran into an issue on our project: the script was catching some files even though they were exlucded in .rubocop.yml, since running rubocop $file overrides all exclusions. Adding --force-exclusion helped us:

ERRORS+=("$(rubocop --force-exclusion $file | grep -e 'C:' -e 'E:')")

When there are a lot of files, it would faster to run rubocop once for all changed files:

if git diff HEAD --name-only | grep -q -E '\.rb$'; then
  ERRORS+=("$(git diff HEAD --name-only | grep -E '.rb' | xargs bundle exec rubocop | grep -e 'C:' -e 'E:')")
fi

Here's another version that runs on all the files in parallel and also checks that you don't have unstaged changes to files you are committing, as that can lead to rubocop passing when it really shouldn't (like if you fix a violation but forget to stage the change):

https://gist.github.com/mckeed/baa6ce5d85b28ef5690bfa025890fb0d