fix for rebasing

These commits should fix some issues I was having with svn2git --rebase including that at least for me if one created another remote, rebasing would start creating a mess thinking that the remotes were in SVN.  As such, I prefixed all the svn remotes with "svn" so that it would be easy to separate svn remotes from others.

Commit message from first commit:
Attempt to fix problems with svn2git --rebase.

- Should no longer get confused by other remotes being added (so long as they are not named "svn")
- Should update branches from svn when svn2git --rebase is invoked

WARNING: Adds "svn" prefix to all remotes from svn, and later assumes this prefix when doing updates.  --rebase won't work with previously existing repositories that don't have this.