move identity config parsing to ident.c

move identity config parsing to ident.c

There's no reason for this to be in config, except that once
upon a time all of the config parsing was there. It makes
more sense to keep the ident code together.

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