only check style on lines changed by commit

bin/git-pre-commit-hook

@@ -9,14 +9,9 @@
 
 cd $(dirname "$0")/../..
 
-IFS=$'\n'
-files=( $(git diff-index --cached --name-only --diff-filter=AMCR HEAD) )
+errors=$(git diff --cached | bin/style-check --diff 2>&1)
 
-if [ "${#files}" -gt 0 ]; then
-    errors=$(bin/style-check "${files[@]}" 2>&1)
-
-    if [ "$?" != "0" ]; then
-        echo "$errors"
-        exit 1
-    fi
+if [ "$?" != "0" ]; then
+    echo "$errors"
+    exit 1
 fi

bin/style-check

@@ -2,5 +2,7 @@
 
 # Checks Python coding style based on our project's preferences.  Checks the
 # files passed on the command-line or everything if no files are passed.
+# Instead of files, "--diff" may be passed to check only the lines changed
+# by a diff piped to standard input.
 
 flake8 --count --max-complexity=10 --max-line-length=150 --statistics --exclude=pyembroidery,__init__.py,electron,build,src,dist "${@:-.}"