pull --rebase: honor rebase.autostash when fast-forwarding

pull --rebase: honor rebase.autostash when fast-forwarding

"pull --rebase" internally uses the merge machinery when the other
history is a descendant of ours (i.e. perform fast-forward).  This
came from [1], where the discussion was started from a feature
request to do so.  It is a bit hard to read the rationale behind it
in the discussion, but it seems that it was an established fact for
everybody involved that does not even need to be mentioned that
fast-forwarding done with "rebase" was much undesirable than done
with "merge", and more importantly, the result left by "merge" is as
good as (or better than) that by "rebase".

Except for one thing.  Because "git merge" does not (and should not)
honor rebase.autostash, "git pull" needs to read it and forward it
when we use "git merge" as a (hopefully better) substitute for "git
rebase" during the fast-forwarding.  But we forgot to do so (we only
add "--[no-]autostash" to the "git merge" command when "git pull" itself
was invoked with "--[no-]autostash" command line option.

Make sure "git merge" is run with "--autostash" when
rebase.autostash is set and used to fast-forward the history on
behalf of "git rebase".  Incidentally this change also takes care of
the case where

 - "git pull --rebase" (without other command line options) is run
 - "rebase.autostash" is not set
 - The history fast-forwards

In such a case, "git merge" is run with an explicit "--no-autostash"
to prevent it from honoring merge.autostash configuration, which is
what we want.  After all, we want the "git merge" to pretend as if
it is "git rebase" while being used for this purpose.

[1] https://lore.kernel.org/git/xmqqa8cfbkeq.fsf_-_@gitster.mtv.corp.google.com/

Reported-by: Tilman Vogel <tilman.vogel@web.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>