feat: Use mirror git cloning

Readme.md

@@ -36,6 +36,12 @@ Example config is available in [config.example.toml](config.example.toml).
 Use `main.api_token` for Gitea API access. Use `fetch.ssh_key` for SSH
 cloning, or `fetch.user` and `fetch.token` for HTTP(S) cloning.
 
+Repositories are stored as bare mirror clones. On the first run the tool uses
+`git clone --mirror <url> <path>`, and on following runs it updates each mirror
+with `git remote update --prune`. This mirrors all refs and handles forced
+updates from the server side, but the resulting directories are not working
+tree checkouts.
+
 
 ### Native
 Not recommended, but more efficient in space