Delegate modified_lines_in_file to context

[Quintasan]

@sds This went under the radar since modified_files is delegated via Overcommit::Hook::Base.

Now that I tried writing a custom hook I noticed modified_lines_in_file is not available inside Overcommit::Hook::CommitMsg::Base. I decided to expose only the method I need but I think it's worth thinking about if we want to delegate all methods that come from included modules by default

[Quintasan]

@sds ding

[sds]

Thanks for the ping.