Support flipped externals with a caret in them

[reversefold]

In a repo at work I found some svn:externals which have the URL first and the directory second. Not only this, the urls start with ^, which is supposed to be "Repository Root". Lastly, these started with "../" to go up past the top of the current repo and then had a directory after that to go back down to another repo.

I've added support for swapping the externals based on a "slash" being in the remote url.

When a "^" is found, it and everything before it is replaced with the "Repository Root".

When "/../" is found, it and the directory before it are removed.

---

This change is 

[reversefold]

ping

[SvenRieke]

Just tried it and works fine.