Repository are not properly mirrored

[pgrenaud]

If we look at the source, the repo are cloned using only the --bare option, and updated using fetch --all --prune and using customs refs.

https://github.com/eXolnet/redmine_git_mirror/blob/master/lib/redmine_git_mirror/git.rb#L45
https://github.com/eXolnet/redmine_git_mirror/blob/master/lib/redmine_git_mirror/git.rb#L64
https://github.com/eXolnet/redmine_git_mirror/blob/master/lib/redmine_git_mirror/git.rb#L52-L57
https://github.com/eXolnet/redmine_git_mirror/blob/master/lib/redmine_git_mirror/git.rb#L79-L101

Looking at the git man pages, git clone have an option --mirror specifically made for this. And the same section mention using git remote update to sync the mirror (which can be run with --prune).

https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---mirror

Looking at this answer, git remote update seems more appropriate over git fetch to sync a mirror.

https://stackoverflow.com/a/17712553

Therefore, we should probably switch to git clone --mirror and git remote update --prune and drop the customs refs. That way, we should safely assume that the repository are a complete mirror of the source repository.