ident: split setup_ident into separate functions

ident: split setup_ident into separate functions

This function sets up the default name, email, and date, and
is not publicly available. Let's split it into three public
functions so that callers can get just the parts they need.

While we're at it, let's change the interface to simple
accessors. The original function was called only by fmt_ident,
and contained logic for "if we already have some other
value, don't load the default" which properly belongs in
fmt_ident.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>