In my SkyDrive I have uploaded the SVN-mirror with full history of NHibernate trunk.
It is updated to 4985 revision. You can use it as you need.
To update it the command example is:
D:\svnmirrors>svnsync sync file:///d:/svnmirrors/NHibernateMirror
When you have an updated revision, you can use it to create your own fork where you need.
To convert it to Mercurial what you need is only the last steps explained in this blog post.
To download it click below.