hooks/post-receive-email: use plumbing instead of git log/show

hooks/post-receive-email: use plumbing instead of git log/show

This way the hook doesn't have to keep being tweaked as porcelain
learns new features like color and pagination.

While at it, replace the "git rev-list | git shortlog" idiom with
plain "git shortlog" for simplicity.

Except for depending less on the value of settings like '[log]
abbrevCommit', no change in output intended.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>