Remove duplication in *Backup

[christianspecht]

There are multiple places like this:

   public override void BackupRepo(string subdir, ScmCredentials credentials)
    {
        InitScm();
        scm.PullFromRemote(this.repo.CloneUrl, subdir, credentials);

        if (!scm.DirectoryIsRepository(subdir))
        {
            throw new InvalidOperationException(Resource.DirectoryNoRepo);
        }
    }

There's already a "default" method in df37341, but it needs to be called in:

• https://github.com/christianspecht/scm-backup/blob/master/src/ScmBackup/Hosters/Github/GithubBackup.cs
• https://github.com/christianspecht/scm-backup/blob/master/src/ScmBackup/Hosters/Bitbucket/BitbucketBackup.cs