remove magit-svn.el, magit-stgit.el, magit-topgit.el and bin/magit

I have removed the libraries `magit-svn.el`, `magit-stgit.el` and `magit-topgit.el`, as well as the `magit` shell-script from the `next` branch of the Magit repository, and have instead created a separate repository for each one of them.

This is something I wanted to do for a long long time, but I have delayed doing it until the time was right.  Now that most of the refactoring is done this time has come.

These libraries/packages haven't changed much in recent years, in fact they have not changed much at all ever since being added to Magit.  I think this is partly due to the fact that the authors didn't contribute much after the libraries were merged into Magit and that the Magit maintainers did not use these libraries enough (except for testing I haven't used them at all) to have a sensitive to improving them.

In recent months I have fixed bugs that were reported by users and heavily cleaned up the code a lot but mostly without adding any new features.  And I will continue to adapt these files as changes in Magit make this necessary and also fix bugs; but I have no interest in adding any new features.

**By creating individual repositories for these packages I hope to encourage people who actually use them to become the new maintainers. Please contact me if you are interested.**

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Instead of using `git filter-branch` to limit the history to just the appropriate file, I have created "incarnation commit"s which are like "initial commit"s except that they contain instructions on how to attach old history if needed.