Git pre-commit hook is slow

[TravisCarden]

The Git pre-commit hooks is very slow. I was prepared to tolerate it for ordinary commits, but when I saw that it fires for every commit on a rebase I realized it was unacceptable. Looking at the file I see we're linting custom Twig files and validating the composer.json file whether or not those files have been changed. We should at least make those steps conditional. Perhaps there's a way to optimize phpcs, as well.

[grasmash]

Modify https://github.com/acquia/blt/blob/8.6.11/scripts/git-hooks/pre-commit:

• Execute validate:twig only if *twig files are listed in $LIST
• Execute composer validate only if composer.* files are listed in $LIST

[TravisCarden]

Yay! Thanks, @grasmash and @malikkotob!