Only gets email addresses from commits reachable from the current head.

Add --all to the git log command to get a more complete view of things.

There's also --walk-reflogs if you want to find output from commits not currently reachable from any branches (although that's not likely relevant for a clean clone operation) but this is mutually exclusive with other options and so the output needs to be combined with the output of git log --all to be truly comprehensive.