Enable to save Git-TF configuration parameters in the global git config file.

    With this change:
        1. The 'git tf clone' command does not store in the local repository config file 'depth', 'tag', and 'include-metadata' parameters. The user has to configure them explicitly with 'git tf configure' or 'git config' commands.
        2. The 'git tf configure' command puts to the the local repository config file _only_ the parameters explicitly specified by the user.
        3. The user has an ability to specify git-tf configuration parameters in a global .gitconfig file by using commands:
             git config --global git-tf.depth {1|number}  - where number is any positive number between 2 and 2147483647.
             git config --global git-tf.tag {true|false}
             git config --global git-tf.include-metadata {true|false}
    In the similar way these parameters could be specified in the --system git configuration.