ident: make xgetpwuid_self() a static local helper

ident: make xgetpwuid_self() a static local helper

This function is defined in wrapper.c, but nobody besides
ident.c uses it. And nobody is likely to in the future,
either, as anything that cares about the user's name should
be going through the ident code.

Moving it here is a cleanup of the global namespace, but it
will also enable further cleanups inside ident.c.

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