Merge branch 'jk/maint-commit-amend-only-no-paths' into maint

"git commit --amend --only --" was meant to allow "Clever" people to
rewrite the commit message without making any change even when they
have already changes for the next commit added to their index, but
it never worked as advertised since it was introduced in 1.3.0 era.

* jk/maint-commit-amend-only-no-paths:
  commit: fix "--amend --only" with no pathspec

builtin/commit.c

@@ -184,6 +184,9 @@ static int list_paths(struct string_list *list, const char *with_tree,
 	int i;
 	char *m;
 
+	if (!pattern)
+		return 0;
+
 	for (i = 0; pattern[i]; i++)
 		;
 	m = xcalloc(1, i);
@@ -345,7 +348,7 @@ static char *prepare_index(int argc, const char **argv, const char *prefix,
 	 * and create commit from the_index.
 	 * We still need to refresh the index here.
 	 */
-	if (!pathspec || !*pathspec) {
+	if (!only && (!pathspec || !*pathspec)) {
 		fd = hold_locked_index(&index_lock, 1);
 		refresh_cache_or_die(refresh_flags);
 		if (active_cache_changed) {

t/t7501-commit.sh

@@ -108,6 +108,16 @@ test_expect_success 'amend commit' '
 	EDITOR=./editor git commit --amend
 '
 
+test_expect_success 'amend --only ignores staged contents' '
+	cp file file.expect &&
+	echo changed >file &&
+	git add file &&
+	git commit --no-edit --amend --only &&
+	git cat-file blob HEAD:file >file.actual &&
+	test_cmp file.expect file.actual &&
+	git diff --exit-code
+'
+
 test_expect_success 'set up editor' '
 	cat >editor <<-\EOF &&
 	#!/bin/sh