git-svnversion

#!/bin/sh
# klg, Apr 2016
set -e

UPSVN="$(git log -n 1 --format=%H --grep '^git-svn-id:' -- "$@")"

if [ -z "$UPSVN" ]; then
  echo >&2 "No git-svn commit found"
  exit 1
fi

REV=$(git log -n 1 --format=%B "$UPSVN" | \
  sed -n 's/^git-svn-id:.*@//p' | sed 's/ .*//')
if [ -n "$(git log --oneline "$UPSVN"..)" ]; then
  REV="${REV}M"
fi

echo "$REV"

This is good for git svn on Linux etc, but not on Windows.