[ruby] rubocop, but only check git staged files

lint-staged.sh

#!/bin/sh

# time execution
time bin/rubocop -L --stderr $(git diff --name-only --cached | grep '\.rb')

# bare minimum
bin/rubocop $(git diff --name-only --cached | grep '\.rb')

# -L = list files (good to see which one got checked since we don't see the nested command's output
# --stderr = may or may not be useful to pipe output there

# @see <https://docs.rubocop.org/rubocop/usage/basic_usage.html#command-line-flags>

# add other flags as you see fit