2.36 fast-export regression fix

e900d494dc (diff: add an API for deferred freeing, 2021-02-11) added a
way to allow reusing diffopts: the no_free bit.  244c27242f (diff.[ch]:
have diff_free() call clear_pathspec(opts.pathspec), 2022-02-16) made
that mechanism mandatory.

git fast-export doesn't set no_free, so path limiting stopped working
after the first commit.  Set the flag and add a basic test to make sure
only changes to the specified files are exported.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

@@ -1261,6 +1261,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 	revs.diffopt.format_callback = show_filemodify;
 	revs.diffopt.format_callback_data = &paths_of_changed_objects;
 	revs.diffopt.flags.recursive = 1;
+	revs.diffopt.no_free = 1;
 	while ((commit = get_revision(&revs)))
 		handle_commit(commit, &revs, &paths_of_changed_objects);
 

t/t9350-fast-export.sh

@@ -500,6 +500,13 @@ test_expect_success 'path limiting with import-marks does not lose unmodified fi
 	grep file0 actual
 '
 
+test_expect_success 'path limiting works' '
+	git fast-export simple -- file >actual &&
+	sed -ne "s/^M .* //p" <actual | sort -u >actual.files &&
+	echo file >expect &&
+	test_cmp expect actual.files
+'
+
 test_expect_success 'avoid corrupt stream with non-existent mark' '
 	test_create_repo avoid_non_existent_mark &&
 	(