How to convert a normal Git repository to a bare one?

How can I convert a 'normal' Git repository to a bare one?

The main difference seems to be:

in the normal Git repository, you have a .git folder inside the repository containing all relevant data and all other files making up your working copy
in a bare Git repository, there is no working copy and the folder (let's call it repo.git) contains the actual repository data

Solution

In short: replace the contents of repo with the contents of repo/.git, then tell the repository that it is now a bare repository.

To do this, execute the following commands:

cd repo
mv .git ../repo.git # renaming just for clarity
cd ..
rm -fr repo
cd repo.git
git config --bool core.bare true

Note that this is different from doing a git clone --bare /path/to/repo to a new location (as described here).

Your origin had 100 branches
- You have only checked out 10 of them locally
- Your bare repo will only have the 10
You send the bare repo somewhere
- It would be missing 90 branches

If that's your intention, that's fine. If you needed a mirror of the remote/origin, this is not the way.