Previous one warned people upfront to encourage fixing their
environment early, but some people just use repositories and git
tools read-only without making any changes, and in such a case
there is not much point insisting on them having a usable ident.

This round attempts to move the error until either "git-var"
asks for the ident explicitly or "commit-tree" wants to use it.

Signed-off-by: Junio C Hamano <junkio@cox.net>