ident: check for useConfigOnly before auto-detection of name/email

ident: check for useConfigOnly before auto-detection of name/email

If user.useConfigOnly is set, it does not make sense to try to
auto-detect the name and/or the email.  The auto-detection may
even result in a bogus name and trigger an error message.

Check if the use-config-only is set and die if no explicit name was
given, before attempting to auto-detect, to correct this.

Signed-off-by: Marios Titas <redneb@gmx.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>