move git_default_* variables to ident.c

move git_default_* variables to ident.c

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