Update git hook tests

[markdboyd]

Changes proposed in this pull request:

Addresses #37

As noted in #37, it is possible to have local git hooks and gitleaks co-exist, but the tests run by make audit assume that is not possible. To fix this behavior, this PR:

• Adds a test to prove that local git hooks and gitleaks both run on a test repo
• Removes the test that fails make audit if a repo has a custom .git/hooks/pre-commit hook, since this scenario does not actually prevent gitleaks from running
• Adds a test to prove that core.hooksPath is not overridden at the repo level, which would prevent gitleaks from running

Other unrelated fixes in the PR include:

• Updates to README to remove outdated information
• Refactor shell scripts based on shellcheck recommendations

security considerations

This PR should be correcting a misunderstanding of how git hooks work but not actually introducing any chance that the global git hook for gitleaks won't run. However, the changes should be audited carefully to avoid any possible regression

[bengerman13]

thanks for cleaning up the README, too!