annex: Hide `git-annex` and `synced/*` branches

[kousu]

The 'git-annex' and 'synced/' branches should be special-cased so that they don't show up as pull request suggestions, either on the command line after a push or on the web UI.

Accidentally merging git-annex into main would be a disaster and we should prevent it.

[kousu]

Relatedly, these branches should not trigger webhooks. They should be invisible, except to git pull and git push.

The route to view them (/{user}/{repo}/src/branch/{branch}/...) should give 404 (or 403?).

Is there any other feature they need to be hidden from?

[kousu]

Also here in the messages returned over ssh:

push origin 
Énumération des objets: 294, fait.
Décompte des objets: 100% (294/294), fait.
Compression par delta en utilisant jusqu'à 128 fils d'exécution
Compression des objets: 100% (133/133), fait.
Écriture des objets: 100% (152/152), 12.68 Kio | 1.58 Mio/s, fait.
Total 152 (delta 103), réutilisés 0 (delta 0), réutilisés du pack 0
remote: Résolution des deltas: 100% (103/103), complété avec 97 objets locaux.
remote: 
remote: Create a new pull request for 'synced/git-annex':
remote:   http://localhost:3000/kousu/spine-generic-processed/compare/master...synced/git-annex
remote: 
remote: . Processing 1 references
remote: Processed 1 references in total
To localhost:kousu/spine-generic-processed.git
   ecc971f37..316a4ea11  git-annex -> synced/git-annex

[mguaypaq]

That would mean that these branches can only be manipulated from the command-line, which is fine I guess. Also, it will be impossible to suggest a fix to someone else's git-annex branch without having write access to the repository. But we can live with that, right?