git-impact.sh

#!/bin/bash

# use newline as separator
IFS=$'\n'

for author in $(git log --all --format='%cN' | sort -u); do
    echo $author; 
    git log --all --author="$author" --pretty=tformat: --numstat | awk '{ add += $1; subs += $2; loc += $1 - $2 } END { printf "++ %s\n-- %s\n= %s\n", add, subs, loc }' -
done

# git config --global alias.impact '!sh git-impact.sh'

Add this to your bin directory where git is located (/usr/bin)
or alias it in git as shown
Then run git impact

to display stats like

Firstname Lastname
+ 8247
- 658
= 7589

zsh formatted

#!/bin/bash

# use newline as separator
IFS=$'\n'

for author in $(git log --all --format='%cN' | sort -u); do
    echo "\033[1;37m$author\033[0m"; 
    git log --all --author="$author" --pretty=tformat: --numstat | awk '{ add += $1; subs += $2; loc += $1 - $2 } END { printf "\033[0;32m++ %s\n\033[0;31m-- %s\n\033[0m= %s\n", add, subs, loc }' -
done

# git config --global alias.impact '!sh git-impact.sh'