How do I tell git-svn about a remote branch created after I fetched the repo?

If you want to track ALL the remote svn branches, then the solution is as simple as:

git svn fetch

This will fetch ALL the remote branches that have not been fetched yet.

Extra tip: if you checked out only the trunk at first, and later you want to track ALL branches, then edit .git/config to look like this and re-run git svn fetch:

[svn-remote "svn"]
        url = https://svn/path_to_repo_root/
        fetch = path_to_trunk:refs/remotes/git-svn
        branches = path_to_branches/*:refs/remotes/*

The key points are url should point to the repository root, and the paths defined in fetch and branches should be relative to url.

If you want to fetch only specific branches instead of ALL, there is a nice example in git svn --help:

[svn-remote "huge-project"]
        url = http://server.org/svn
        fetch = trunk/src:refs/remotes/trunk
        branches = branches/{red,green}/src:refs/remotes/branches/*
        tags = tags/{1.0,2.0}/src:refs/remotes/tags/*

With older versions of git-svn, once you specified branches like this, you might not be able to get new branches with git svn fetch. One workaround is adding more fetch lines, like this:

[svn-remote "huge-project"]
        url = http://server.org/svn
        fetch = trunk/src:refs/remotes/trunk
        fetch = branches/blue:refs/remotes/branches/blue
        fetch = branches/yellow:refs/remotes/branches/yellow
        branches = branches/{red,green}/src:refs/remotes/branches/*

Another workaround by @AndyEstes: edit .git/svn/.metadata and change the value of branches-maxRev or tags-maxRev to a revision before any newly-specified branches or tags were created. Once you've done this, run git svn fetch to track the new svn remote branch.

You can manually add the remote branch,

git config --add svn-remote.newbranch.url https://svn/path_to_newbranch/
git config --add svn-remote.newbranch.fetch :refs/remotes/newbranch
git svn fetch newbranch [-r<rev>]
git checkout -b local-newbranch -t newbranch
git svn rebase newbranch