feat(astyle): add option to only format diff with respect to HEAD (#27122)

* feat(astyle): add make format_changed for diff only style fixes

`make format` is pretty slow because it always considers the entire
source tree. For developing on top of a clean state, considering only
files that differ from HEAD should be sufficient.

* docs(astyle): document new format_changed target

Tools/astyle/check_code_style.sh

@@ -16,7 +16,7 @@ if [ -n "$CHECK_FAILED" ]; then
 	if [[ $PX4_ASTYLE_FIX -eq 1 ]]; then
 		${DIR}/fix_code_style.sh $FILE
 	else
-		echo 'to fix automatically run "make format" or "./Tools/astyle/fix_code_style.sh' $FILE'"'
+		echo 'to fix automatically run "make format", "make format_changed" or "./Tools/astyle/fix_code_style.sh' $FILE'"'
 		exit 1
 	fi
 fi

Tools/astyle/check_code_style_all.sh

@@ -36,9 +36,13 @@ fi
 CI="${CI:-false}"
 DIR=$(cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd)
 
-if [[ "$@" == "--fix" ]]; then
-    export PX4_ASTYLE_FIX=1
-fi
+DIFF_ONLY=false
+for arg in "$@"; do
+    case "$arg" in
+        --fix) export PX4_ASTYLE_FIX=1 ;;
+        --diff-only) DIFF_ONLY=true ;;
+    esac
+done
 
 
 # install git pre-commit hook
@@ -59,7 +63,24 @@ if [[ ! -f "$HOOK_FILE" && "$CI" != "true" && $- == *i* ]]; then
 	fi
 fi
 
-${DIR}/files_to_check_code_style.sh | xargs -P 8 -I % ${DIR}/check_code_style.sh %
+FILE_LIST=$(${DIR}/files_to_check_code_style.sh)
+
+if [ "$DIFF_ONLY" = true ]; then
+    # --diff-filter=d: do not list deleted files (no need to format)
+    CHANGED=$(git -C "$DIR/../.." diff --name-only --diff-filter=d HEAD -- '*.c' '*.h' '*.cpp' '*.hpp')
+    if [ -z "$CHANGED" ]; then
+        FILE_LIST=""
+    else
+        FILE_LIST=$(echo "$FILE_LIST" | grep -Fx -f <(echo "$CHANGED") || true)
+    fi
+fi
+
+if [ -z "$FILE_LIST" ]; then
+    echo "No files to check"
+    exit 0
+fi
+
+echo "$FILE_LIST" | xargs -P 8 -I % ${DIR}/check_code_style.sh %
 
 if [ $? -eq 0 ]; then
     echo "Format checks passed"