WordPress: 3-way-development with Git without 3rd Party Software - wordpress

Question

Maybe this question is too broad (I hope not) but I'm currently don't know, how to solve my WordPress Git Branching Model problem.
My plan was to create a branching model like this without using a BitBucket Server or Jenkins:
So I've started making a git init in the wp-content folder of my PROD (at this time only installation of WP) system.
After I was done with this, I've cloned the whole PROD page and created a staging subdomain. After cloning was completed, I've deleted the wp-content folder at the staging system and typed in a git clone <prod-path>/.git.
This worked great and after doing some changes (for testing purposes) at the staging version, I was able to push them back to the PROD. Until now, everything is logic and normal and worked very well.
After thinking some time, I've got the great (maybe stupid) idea, to do the same with a local DEV clone on my PC. So I've did the same like at the staging version before, but this time, I've cloned the staging wp-content clone (PROD clone) to my local DEV version.
For the first view, everything looked fine but after trying to push something to the Staging system, I've got rejected from the target staging Git:
remote: error: refusing to update checked out branch: refs/heads/development
remote: error: By default, updating the current branch in a non-bare repository
So I first thought, that it's very stupid what I did, because the staging is also a clone without a HEAD like on the PROD (I think).
So my question to you:
Do you have an idea, how I can keep this kind of Workflow without any 3rd party software, but with 2 clones that I can work on DEV, test on Staging and going live on PROD?
To say it with commands:
1. git push <to the staging>
2. git push <to the prod>
3. git merge <dev to master>
Update
This is the content of my bare hooks folder (there is no post-received, what should I do?):
/wp-content/.bare/hooks$ ls -la
total 56
drwxr-xr-x 2 wp_ftp psacln 4096 Apr 11 13:09 .
drwxr-xr-x 7 wp_ftp psacln 4096 Apr 11 13:20 ..
-rwxr-xr-x 1 wp_ftp psacln 478 Apr 11 13:09 applypatch-msg.sample
-rwxr-xr-x 1 wp_ftp psacln 896 Apr 11 13:09 commit-msg.sample
-rwxr-xr-x 1 wp_ftp psacln 3327 Apr 11 13:09 fsmonitor-watchman.sample
-rwxr-xr-x 1 wp_ftp psacln 189 Apr 11 13:09 post-update.sample
-rwxr-xr-x 1 wp_ftp psacln 424 Apr 11 13:09 pre-applypatch.sample
-rwxr-xr-x 1 wp_ftp psacln 1642 Apr 11 13:09 pre-commit.sample
-rwxr-xr-x 1 wp_ftp psacln 1492 Apr 11 13:09 prepare-commit-msg.sample
-rwxr-xr-x 1 wp_ftp psacln 1348 Apr 11 13:09 pre-push.sample
-rwxr-xr-x 1 wp_ftp psacln 4898 Apr 11 13:09 pre-rebase.sample
-rwxr-xr-x 1 wp_ftp psacln 544 Apr 11 13:09 pre-receive.sample
-rwxr-xr-x 1 wp_ftp psacln 3610 Apr 11 13:09 update.sample

Answer 1

but after trying to push something to the Staging system, I've got rejected from the target staging Git
It depends what is the "rejected" message.
If it is
remote: error: refusing to update checked out branch: refs/heads/development
remote: error: By default, updating the current branch in a non-bare repository
That means you need to have a subfolder "bare" within your staging repo:
cd staging
git init --bare . bare
echo /bare/>>.gitignore
git add .gitignore
git commit -m "Ignore nested bare repo /bare/"
git remote add bare bare
git config remote.bare.fetch '+refs/heads/*:refs/remotes/bare/*'
cd bare
git config remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*'
Then modify the bare/hooks/post-receive file in order for it to:
be executable
include the following content
post-receive hook in bare nested bare repository:
#!/bin/bash
unset GIT_DIR
cd ..
git --work-tree=. --git-dir=.git fetch bare
git --work-tree=. --git-dir=.git fetch --tags bare
if ! git --work-tree=. --git-dir=.git merge-base --is-ancestor master origin/master; then
echo "Reset non-bare master to bare master (forced update)"
git --work-tree=. --git-dir=.git reset --hard origin/master
else
echo "Update non-bare master with merge from origin/master"
git --work-tree=. --git-dir=.git pull --no-rebase origin master
fi
Finally, on your dev cloned repo:
cd dev
git config --unset-all remote.origin.pushurl
git config --add remote.origin.pushurl "/path/to/staging/bare"
That way, you are fetching from the Staging repo as usual, but are pushing to the bare nested Staging repo, and the hook will update said Staging content.