2.36 fast-export regression fix

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>