a.sh

#!/bin/sh -eu
mkcommit() {
    echo "$1" > "$1"; git add "$1"; git commit -m "$1"
}
git_log() {
    git --no-pager log --graph --oneline --all
}

rm -rf a a.git a2.git
mkdir a
(cd a
git init
mkcommit ma
)
git clone --bare a a.git
git clone --bare a a2.git
rm -rf a

mkdir a
(cd a
git init
git remote add r ../a.git
git fetch r
# git remote add origin ../a2.git
git reset --hard r/master
git config push.default current

mkcommit mb
set -x
git push
cat .git/config
)

a.sh (output)

...
+ git push
Enumerating objects: 4, done.
Counting objects: 100% (4/4), done.
Delta compression using up to 8 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 265 bytes | 265.00 KiB/s, done.
Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
To ../a.git
   ec99458..c611b6b  master -> master
+ cat .git/config
[core]
        repositoryformatversion = 0
        filemode = true
        bare = false
        logallrefupdates = true
[remote "r"]
        url = ../a.git
        fetch = +refs/heads/*:refs/remotes/r/*
[push]
        default = current