git pull --rebase=preserve is always rebasing something, even on up-to-date branch

[Mikhail Matrosov <mikhail.matrosov@gmail.com>]

Please try the following:

mmatrosov@Mikhail-PC:~/test$ git init --bare server
Initialized empty Git repository in /home/mmatrosov/test/server/
mmatrosov@Mikhail-PC:~/test$ git clone server local
Cloning into 'local'...
warning: You appear to have cloned an empty repository.
done.
mmatrosov@Mikhail-PC:~/test$ cd local
mmatrosov@Mikhail-PC:~/test/local$ echo a > a && git add . && git commit -m A
[master (root-commit) a34c21f] A
 1 file changed, 1 insertion(+)
 create mode 100644 a
mmatrosov@Mikhail-PC:~/test/local$ git push
Counting objects: 3, done.
Writing objects: 100% (3/3), 205 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To /home/mmatrosov/test/server
 * [new branch]      master -> master
mmatrosov@Mikhail-PC:~/test/local$ git pull
Already up-to-date.
mmatrosov@Mikhail-PC:~/test/local$ git pull --rebase=preserve
Rebasing (1/1)
Successfully rebased and updated refs/heads/master.

As you can see, running bare "git pull" just tells me everything is up
to date. However, running "git pull --rebase=preserve" triggers
rebasing of something. It wont be a problem if it didn't take
significant time (especially on Windows). Why this rebase happens? It
is completely redundant and slows down the pull operation. Looks like
a bug to me.

Note that it is important to me, because I want to set "git config
--global pull.rebase preserve". But because of this issue, pulling on
an up-to-date repository takes a lot of time. Which is very
frustrating.

Tested with:
* git version 2.19.0.windows.1 in Windows 10 Version 1803
* git version 2.7.4 in Ubuntu 16.04.3 LTS (inside WSL)

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Best regards, Mikhail Matrosov