pull: cleanup autostash check

pull: cleanup autostash check

Currently "git pull --rebase" takes a shortcut in the case a
fast-forward merge is possible; run_merge() is called with --ff-only.

However, "git merge" didn't have an --autostash option, so, when "git
pull --rebase --autostash" was called *and* the fast-forward merge
shortcut was taken, then the pull failed.

This was fixed in commit f15e7cf5cc (pull: ff --rebase --autostash
works in dirty repo, 2017-06-01) by simply skipping the fast-forward
merge shortcut.

Later on "git merge" learned the --autostash option [a03b55530a
(merge: teach --autostash option, 2020-04-07)], and so did "git pull"
[d9f15d37f1 (pull: pass --autostash to merge, 2020-04-07)].

Therefore it's not necessary to skip the fast-forward merge shortcut
anymore when called with --rebase --autostash.

Let's always take the fast-forward merge shortcut by essentially
reverting f15e7cf5cc.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>