feat: enable all optional shellcheck validations

Make shell a little bit safer with:

- add-default-case
- check-extra-masked-returns
- check-set-e-suppressed
- quote-safe-variables
- check-unassigned-uppercase

Although there are some stylistic decisions for uniformity:

- avoid-nullary-conditions
- deprecated-which
- require-variable-braces

scripts/markdown-lint.sh

@@ -8,7 +8,9 @@
 set -eu
 
 command -v git >/dev/null || { echo "Missing program: git" >&2; exit 1; }
-cd "$(git rev-parse --show-toplevel)" || exit 1
+repo_toplevel="$(git rev-parse --show-toplevel)"
+test -d "${repo_toplevel}" || exit 1
+unset repo_toplevel
 ./scripts/requires-program.sh mdl
 
 extra_files_rules="~MD002,~MD012,~MD022,~MD032,~MD041"
@@ -17,24 +19,25 @@ find_tool="$(./scripts/best-program.sh fd fdfind find)"
 if test -n "${1-}"; then
   files=""
   extra_files=""
-  for f in "$@"; do
-    test -f "$f" || continue
+  for f in "${@}"; do
+    test -f "${f}" || continue
     extension="${f##*.}"
-    case "$extension" in
+    case "${extension}" in
       md)
         case "${f}" in
-          .github/*) extra_files="$extra_files $f"; continue;;
+          .github/*) extra_files="${extra_files} ${f}"; continue;;
+          *) ;;
         esac
-        files="$files $f";;
+        files="${files} ${f}";;
       *)
         continue
         ;;
     esac
   done
   if test -n "${extra_files}"; then
-    mdl --rules ${extra_files_rules} ${extra_files}
+    mdl --rules "${extra_files_rules}" ${extra_files}
   fi
-  test -n "$files" || exit 0
+  test -n "${files}" || exit 0
   exec mdl ${files}
 fi
 
@@ -47,9 +50,10 @@ case "${find_tool}" in
     files="$(find . -not -path './.github/*' -type f -name "*.md")"
     extra_files="$(find .github -type f -name "*.md")"
     ;;
+  *) echo "Unsupported find tool" >&2; exit 1;;
 esac
 
 if test -n "${extra_files}"; then
-  mdl --rules ${extra_files_rules} ${extra_files}
+  mdl --rules "${extra_files_rules}" ${extra_files}
 fi
 exec mdl ${files}