Ruby example use staged for git diff

[jaydorsey]

git diff --name-only HEAD master doesn't show the current files that
are staged for changes, only the previously committed changes

I think using --staged is a better example for the docs, because it will
run the command against the files you are committing. I think it better
represents what people expect to happen (the files I'm trying to commit will
have rubocop run against them)

I can throw an example git repo up w/ some examples of why I think this
is a better option to present as a default if that helps, but here's a screenshot:

• I'm on a branch (off of master)
• I had 1 commit where I added test.rb file to the branch
• I have foo_spec.rb staged, and I'm adding it as a commit to the branch

Using HEAD master I only see the test.rb file in the output; so lefthook actually skips running rubocop on foo_spec.rb (which has a violation).

I expected lefthook to fail on the rubocop violation (file is staged, I'm committing it) but it did not, because it only looked at my previous committed file (test.rb)

[Arkweid]

Thank you @jaydorsey you are right!