gettext: always use UTF-8 on native Windows

gettext: always use UTF-8 on native Windows

On native Windows, Git exclusively uses UTF-8 for console output (both
with MinTTY and native Win32 Console). Gettext uses `setlocale()` to
determine the output encoding for translated text, however, MSVCRT's
`setlocale()` does not support UTF-8. As a result, translated text is
encoded in system encoding (as per `GetAPC()`), and non-ASCII chars are
mangled in console output.

Side note: There is actually a code page for UTF-8: 65001. In practice,
it does not work as expected at least on Windows 7, though, so we cannot
use it in Git. Besides, if we overrode the code page, any process
spawned from Git would inherit that code page (as opposed to the code
page configured for the current user), which would quite possibly break
e.g. diff or merge helpers. So we really cannot override the code page.

In `init_gettext_charset()`, Git calls gettext's
`bind_textdomain_codeset()` with the character set obtained via
`locale_charset()`; Let's override that latter function to force the
encoding to UTF-8 on native Windows.

In Git for Windows' SDK, there is a `libcharset.h` and therefore we
define `HAVE_LIBCHARSET_H` in the MINGW-specific section in
`config.mak.uname`, therefore we need to add the override before that
conditionally-compiled code block.

Rather than simply defining `locale_charset()` to return the string
`"UTF-8"`, though, we are careful not to break `LC_ALL=C`: the
`ab/no-kwset` patch series, for example, needs to have a way to prevent
Git from expecting UTF-8-encoded input.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>