Check markdown in files known to git, not all files (nhs-england-tools#158)

<!-- markdownlint-disable-next-line first-line-heading -->
## Description

As currently written, `check=all scripts/githooks/check-markdown-format`
will not only check files intentionally checked in, it will also check
anything under, for instance, `.venv/` or `node_modules/` which are out
of our control.

This change makes it only pay attention to checked-in files. I don't
think it was intentional that we lint markdown in dependencies.

## Type of changes

<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply. -->

- [ ] Refactoring (non-breaking change)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would change existing
functionality)
- [x] Bug fix (non-breaking change which fixes an issue)

## Checklist

<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->

- [x] I am familiar with the [contributing
guidelines](../docs/CONTRIBUTING.md)
- [x] I have followed the code style of the project
- [ ] I have added tests to cover my changes
- [ ] I have updated the documentation accordingly
- [ ] This PR is a result of pair or mob programming

---

## Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others
privacy, we kindly ask you to NOT including [PII (Personal Identifiable
Information) / PID (Personal Identifiable
Data)](https://digital.nhs.uk/data-and-information/keeping-data-safe-and-benefitting-the-public)
or any other sensitive data in this PR (Pull Request) and the codebase
changes. We will remove any PR that do contain any sensitive
information. We really appreciate your cooperation in this matter.

- [x] I confirm that neither PII/PID nor sensitive data are included in
this PR and the codebase changes.

Signed-off-by: Alex Young <[email protected]>

Author: regularfry

scripts/githooks/check-markdown-format.sh

@@ -38,7 +38,7 @@ function main() {
   check=${check:-working-tree-changes}
   case $check in
     "all")
-      files="$(find ./ -type f -name "*.md")"
+      files="$(git ls-files "*.md")"
       ;;
     "staged-changes")
       files="$(git diff --diff-filter=ACMRT --name-only --cached "*.md")"