git log --no-merges --pretty="%an_-_%ae_-_%ad" --date=iso |
awk -F "_-_" '
# count commits by author
{
    commits[$1]++
}

# if first commit, store date
commits[$1] == 1 {
    latestdate[$1] = $3
}

# dump it
END {
    for (i in commits) {
        print i, ": ", commits[i], " Latest: ", latestdate[i]
    }
}
'