Change how check-file-format.sh checks changed files

Prior to this patch, the `git diff` command used to identify which files to check for editorconfig compliance caught too many files, and did the expensive part of the check inside the per-file loop.

By lucky chance, the editorconfig-checker container image has a `git` binary installed, so we can move the entire loop inside a single container invocation.

Fixes #129.

scripts/githooks/check-file-format.sh

@@ -48,18 +48,11 @@ function main() {
   else
 
     # Check changed files only
-    files=$( (git diff --diff-filter=ACMRT --name-only ${BRANCH_NAME:-origin/main}; git diff --name-only) | sort | uniq )
-    if [ -n "$files" ]; then
-      while read file; do
-        docker run --rm --platform linux/amd64 \
-          --volume=$PWD:/check \
-          mstruebing/editorconfig-checker:$image_version \
-            ec \
-              --exclude '.git/' \
-              "$file"
-        [ $? != 0 ] && exit_code=1 ||:
-      done < <(echo "$files")
-    fi
+    docker run --rm --platform linux/amd64 \
+      --volume=$PWD:/check \
+      mstruebing/editorconfig-checker:$image_version \
+        sh -c 'ec --exclude ".git/" $(git diff --diff-filter=ACMRT --name-only)'
+    [ $? != 0 ] && exit_code=1 ||:
 
   fi
 }