Regarding commits, refs, branches and "et cetera", Magnus answer just works (git remote update).
But unfortunately there is no way to clone / mirror / update the hooks, as I wanted... 
I have found this very interesting thread about cloning/mirroring the hooks:
http://kerneltrap.org/mailarchive/git/2007/8/28/256180/thread
I learned:
- The hooks are not considered part of the repository contents. 
- There is more data, like the - .git/descriptionfolder, which does not get cloned, just as the hooks.
 
- The default hooks that appear in the - hooksdir comes from the- TEMPLATE_DIR
 
- There is this interesting - templatefeature on git.
 
So, I may either ignore this "clone the hooks thing", or go for a rsync strategy, given the purposes of my mirror (backup + source for other clones, only).
Well... I will just forget about hooks cloning, and stick to the git remote update way.
- Sehe has just pointed out that not only "hooks" aren't managed by the clone/updateprocess, but also stashes, rerere, etc... So, for a strict backup,rsyncor equivalent would really be the way to go. As this is not really necessary in my case (I can afford not having hooks, stashes, and so on), like I said, I will stick to theremote update.
Thanks! Improved a bit of my own "git-fu"... :-)