Make "empty ident" error message a bit more helpful.

Make "empty ident" error message a bit more helpful.

It appears that some people who did not care about having bogus
names in their own commit messages are bitten by the recent
change to require a sane environment [*1*].

While it was a good idea to prevent people from using bogus
names to create commits and doing sign-offs, the error message
is not very informative.  This patch attempts to warn things
upfront and hint people how to fix their environments.

[Footnote]

*1* The thread is this one.

    http://marc.theaimsgroup.com/?t=113868084800004

    Especially this message.

    http://marc.theaimsgroup.com/?m=113932830015032

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