t5520: make test_pull_autostash() accept expect_parent_num

t5520: make test_pull_autostash() accept expect_parent_num

Before, test_pull_autostash() was hardcoded to run
`test_cmp_rev HEAD^ copy` to test that a rebase happened. However, in a
future patch, we plan on testing merging as well. Make
test_pull_autostash() accept a parent number as an argument so that, in
the future, we can test if a merge happened in addition to a rebase.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>