If you use zsh, feel free to grab https://sourceforge.net/p/zsh/code/ci/04513dd5df31bc235691400b76a524d117cefede/tree/Completion/Unix/Command/_subversion?format=raw and drop it into a directory that's in your $fpath variable (`print -raC1 -- $fpath`). Earlier directories have precedence over later ones.
The file will probably work with zsh as old as 5.3, but I haven't tested all the way back. Bug reports to zsh-us...@zsh.org. Thanks, Daniel