I'm not an expert on git-svn internals, but I believe sub-project
branches are a limitation of its branching implementation. svn-fe
doesn't have this problem at the downloading stage (and will do 20,000
revisions much quicker than git-svn), but requires you to split branches
out by hand.
I'm currently trying to document these strange edge cases as part of an
ongoing effort to add branch information on top of svn-fe's output.
Would you mind taking a look at [1] and letting me know if it's an
understandable description of your problem? Improvements and other edge
cases are very much welcome, and I hope to have something in the coming
months that tackles these issues.
- Andrew
[1]https://github.com/andrew-sayers/SVN-Branching-Language/blob/master/t/advanced/subproject_branch.sh