Question: how is the Gitman cache updated?

Hi! I'm trying to better understand how Gitman's cache system works in practice.

As far as I can tell, the cache is created using:

```bash
git clone --mirror
```

This results in a bare mirror of the original repository stored locally (under `.gitman_cache` or `$GITMAN_CACHE`). This is very useful to speed up subsequent clones and to save bandwidth.

However, I couldn't find any documentation or code that clearly describes how and **when** the cache is refreshed after its first creation.

My questions:

- Is the cache ever updated by Gitman once it's been created?
- Is there any built-in mechanism to update or re-fetch tags/commits from the origin into the cache mirror?
- Or is it up to the user to periodically run something like `git fetch --all` inside `.gitman_cache/.reference`?

Thanks a lot for this great tool, and for any clarification you can provide!

Max

Start by tracing the documented `git clone --mirror` cache creation and inspect the `.gitman_cache/.reference` path mentioned in the issue. Determine whether Gitman refreshes the mirror or requires a user-run fetch, then document the observed cache-update behavior and supported command or workflow.

Written by the indexing model from the issue text.