utf8: teach same_encoding() alternative UTF encoding names
The function same_encoding() could only recognize alternative names for
UTF-8 encodings. Teach it to recognize all kinds of alternative UTF
encoding names (e.g. utf16).
While we are at it, fix a crash that would occur if same_encoding() was
called with a NULL argument and a non-NULL argument.
This function is used in a subsequent commit.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>