If `user.name` and `user.email` have not been configured and the
user invokes:

    git commit --author=...

without specifying the committer identity, then Git errors out with
a message asking the user to configure `user.name` and `user.email`
but doesn't tell the user which attribution was missing. This can be
confusing for a user new to Git who isn't aware of the distinction
between user, author, and committer.

Give such users a bit more help by extending the error message to
also say which attribution is expected.

Signed-off-by: Junio C Hamano <gitster@pobox.com>