utf8: accept alternate spellings of UTF-8

The iconv implementation on many platforms will accept
variants of UTF-8, including "UTF8", "utf-8", and "utf8",
but some do not. We make allowances in our code to treat
them all identically, but we sometimes hand the string from
the user directly to iconv. In this case, the platform iconv
may or may not work.

There are really four levels of platform iconv support for
these synonyms:

  1. All synonyms understood (e.g., glibc).

  2. Only the official "UTF-8" understood (e.g., Windows).

  3. Official "UTF-8" not understood, but some other synonym
     understood (it's not known whether such a platform exists).

  4. Neither "UTF-8" nor any synonym understood (e.g.,
     ancient systems, or ones without utf8 support
     installed).

This patch teaches git to fall back to using the official
"UTF-8" spelling when iconv_open fails (and the encoding was
one of the synonym spellings). This makes things more
convenient to users of type 2 systems, as they can now use
any of the synonyms for the log output encoding.

Type 1 systems are not affected, as iconv already works on
the first try.

Type 4 systems are not affected, as both attempts already
fail.

Type 3 systems will not benefit from the feature, but
because we only use "UTF-8" as a fallback, they will not be
regressed (i.e., you can continue to use "utf8" if your
platform supports it). We could try all the various
synonyms, but since such systems are not even known to
exist, it's not worth the effort.

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

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