utf8: refactor code to decide fallback encoding

utf8: refactor code to decide fallback encoding

The codepath we use to call iconv_open() has a provision to use a
fallback encoding when it fails, hoping that "UTF-8" being spelled
differently could be the reason why the library function did not
like the encoding names we gave it.  Essentially, we turn what we
have observed to be used as variants of "UTF-8" (e.g. "utf8") into
the most official spelling and use that as a fallback.

We do the same thing for input and output encoding.  Introduce a
helper function to do just one side and call that twice.

Signed-off-by: Junio C Hamano <gitster@pobox.com>