mingw: use Unicode functions explicitly

Many Win32 API functions actually exist in two variants: one with the `A` suffix that takes ANSI parameters (`char *` or `const char *`) and one with the `W` suffix that takes Unicode parameters (`wchar_t *` or `const wchar_t *`). The ANSI variant assumes that the strings are encoded according to whatever is the current locale. This is not what Git wants to use on Windows: we assume that `char *` variables point to strings encoded in UTF-8. There is a pseudo UTF-8 locale on Windows, but it does not work as one might expect. In addition, if we overrode the user's locale, that would modify the behavior of programs spawned by Git (such as editors, difftools, etc), therefore we cannot use that pseudo locale. Further, it is actually highly encouraged to use the Unicode versions instead of the ANSI versions, so let's do precisely that. Note: when calling the Win32 API functions _without_ any suffix, it depends whether the `UNICODE` constant is defined before the relevant headers are #include'd. Without that constant, the ANSI variants are used. Let's be explicit and avoid that ambiguity. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>