Thanks for your time to look into that!
After applying your ideas (which seem to work well) I end up with:
==================================================================
git svn init file://"${ABS_SVN_SOURCE}"
git config svn-remote.svn.fetch 'trunk:refs/remotes/trunk@1889'
git svn fetch -r1:1889
git svn init file://"${ABS_SVN_SOURCE}" \
--trunk=main/trunk
git config svn-remote.svn.tags 'main/tags/*:refs/tags/*'
git config svn-remote.svn.branches 'main/branches/*:refs/heads/*'
git svn fetch -r1890:HEAD
==================================================================.git/config holds:
==================================================================
[..]
[svn-remote "svn"]
url = file:///[..]/portage-anon-svn-repo-dump
fetch = trunk:refs/remotes/trunk@1889
fetch = main/trunk:refs/remotes/trunk
tags = main/tags/*:refs/tags/*
branches = main/branches/*:refs/heads/*
==================================================================Thought I'd paste that if anybody else is interested.
Sebastian