Relocating a git-svn mirror of a Subversion repository isn't trivial:
git-svn: what’s the equivalent to svn switch —relocate?
So why doesn't git-svn use the repository's UUID instead of its URL in the git-svn-id? The UUID uniquely identifies the repository, so it seems a natural identifier to use and would handle relocatations much easier.
I don't have an answer to your question, but git-svn uses the repo UUID as well as the URL. Both must be correct for git-svn not to print Unable to determine upstream SVN information from working tree history. I really wish it'd output something like 'repo UUID doesn't match' as I just spent several hours trying to figure this out...
The git wiki has the answer, use the first method documented here:
https://git.wiki.kernel.org/index.php/GitSvnSwitch
I just tested it on a test repo and it seems to work.