There's no reason anybody outside of ident.c should access
these directly (they should use the new accessors which make
sure the variables are initialized), so we can make them
file-scope statics.

While we're at it, move user_ident_explicitly_given into
ident.c; while still globally visible, it makes more sense
to reside with the ident code.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>