Function convert_to_bare 

pub fn convert_to_bare(repo: Repository) -> Result<Repository>

Convert a standard git repository into the worktrees layout.

Performs the following steps:

1. Sets core.bare = true in git config
2. Renames .git to .bare
3. Writes a .git link file containing gitdir: ./.bare
4. Configures origin’s fetch refspec to mirror all branches

Returns the re-opened bare repository.