Delay "empty ident" errors until they really matter.

Delay "empty ident" errors until they really matter.

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>