I was just trying to commit changes to the Git master. From what I have read, it seems that the idea is to delete the lock file. The message says:
make sure no other Git processes are running and remove the file manually
Perhaps someone knows, which file to remove and how to remove?
The file in question is likely .git/index.lock and it should be safe to just remove it if you have no other git processes running. Make sure a git-svn command isn't hanging.
PS My usual approach to fixing git-svn problems is to make a fresh pull of the repository. Time consuming, but you can do it in parallel with trying to fix the problem. Have a little race between you and git. Of course, this only works if you didn't have unpushed commits.