Check markdown in files known to git, not all files

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.

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

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")"