ident.c: check explicit identity for name and email separately

ident.c: check explicit identity for name and email separately

bb1ae3f (commit: Show committer if automatic, 2008-05-04) added a logic to
check both name and email were given explicitly by the end user, but it
assumed that fmt_ident() is never called before git_default_user_config()
is called, which was fragile.  The former calls setup_ident() and fills
the "default" name and email, so the check in the config parser would have
mistakenly said both are given even if only user.name was provided.

Make the logic more robust by keeping track of name and email separately.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Santi Béjar <santi@agolina.net>