By user email:

git ls-files | while read f; do git blame -w -M -C -C --line-porcelain "$f" | grep -I '^author-mail '; done | sort -f | uniq -ic | sort -n

By user name:

git ls-files | while read f; do git blame -w -M -C -C --line-porcelain "$f" | grep -I '^author '; done | sort -f | uniq -ic | sort -n