Support Git repositories as SVN-externals

The .gitsvnextmodules that refer to regular Git repositories are being checked out as SVN, instead of regular Git submodules.

If this Git repository resides on GitHub SmartGit does not support it. (svn: E204900: Can't clone an SVN repository from GitHub. GitHub uses a non-standard SVN server and hence those repositories can't be cloned with SmartGit.)

Also there is no option to add a remote Git submodule via the GUI and the option for a local Git or Mercurial repository is greyed out.

However, adding a remote Git submodule to a SVN-Git bridge checkout via command line works and SmartGit picks it up OK.