Let me steal the idea of no incessant lookups from https://stackoverflow.com/a/79658213/7976758 by @jthill:

#! /bin/sh

git rev-list --no-commit-header --format="%H %ae" HEAD |
    while read commit_ID _email; do
        if [ -z "$first_email" ]; then
            first_email=$_email
        fi
        if [ "$first_email" = "$_email" ]; then
            echo $commit_ID
        else
            exit
        fi
    done