Please add instructions to the git-svn man page concerning cloning a SVN repositiory through a proxy. In particular make clear that also the SVN software has to be configured to use the proxy. See e.g. http://stackoverflow.com/questions/9057111/git-svn-ignores-http-proxy-debian.
I'd prepare a patch, but I have no clue about SVN, so I'm probably not the best choice for writing a SVN related manual. ;)
Kind regards, - lars.