ident.c: reword error message when the user name cannot be determined

ident.c: reword error message when the user name cannot be determined

The "config --global" suggested in the message is a valid one-shot fix,
and hopefully one-shot across machines that NFS mounts the home directories.

This knowledge can hopefully be reused when you are forced to use git on
Windows, but the fix based on GECOS would not be applicable, so
it is not such a useful hint to mention the exact reason why the
name cannot be determined.

Signed-off-by: Santi Béjar <sbejar@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>