Win32: Make sure error messages are consistently UTF-8 encoded

W/o this a libgit2 error message could have a mixed encoding:
e.g. a filename in UTF-8 combined with a native Windows error message
encoded with the local code page.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

src/errors.c

@@ -52,16 +52,25 @@ void giterr_set(int error_class, const char *string, ...)
 	if (error_class == GITERR_OS) {
 #ifdef GIT_WIN32
 		if (win32_error_code) {
-			char *lpMsgBuf;
-
-			if (FormatMessageA(
+			LPWSTR lpMsgBuf = NULL;
+			int size = FormatMessageW(
 					FORMAT_MESSAGE_ALLOCATE_BUFFER |
 					FORMAT_MESSAGE_FROM_SYSTEM |
 					FORMAT_MESSAGE_IGNORE_INSERTS,
-					NULL, win32_error_code, 0, (LPSTR)&lpMsgBuf, 0, NULL)) {
+					NULL, win32_error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+					(LPWSTR)&lpMsgBuf, 0, NULL);
+
+			if (size) {
+				int utf8_size = size * 4 + 1;
+
+				char *lpMsgBuf_utf8 = git__calloc(utf8_size, sizeof(char));
+				GITERR_CHECK_ALLOC(lpMsgBuf_utf8);
+				WideCharToMultiByte(CP_UTF8, 0, lpMsgBuf, size, lpMsgBuf_utf8, utf8_size, NULL, NULL);
+
 				git_buf_PUTS(&buf, ": ");
-				git_buf_puts(&buf, lpMsgBuf);
+				git_buf_puts(&buf, lpMsgBuf_utf8);
 				LocalFree(lpMsgBuf);
+				git__free(lpMsgBuf_utf8);
 			}
 
 			SetLastError(0);

src/netops.c

@@ -40,16 +40,20 @@
 #ifdef GIT_WIN32
 static void net_set_error(const char *str)
 {
-	int size, error = WSAGetLastError();
-	LPSTR err_str = NULL;
+	int error = WSAGetLastError();
+	LPWSTR err_str = NULL;
 
-	size = FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
-			     0, error, 0, (LPSTR)&err_str, 0, 0);
+	int size = FormatMessageW(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
+							  0, error, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPWSTR)&err_str, 0, 0);
 
-	GIT_UNUSED(size);
+	int utf8_size = size * 4 + 1;
+	char * err_str_utf8 = git__calloc(utf8_size, sizeof(char));
+	GITERR_CHECK_ALLOC(err_str_utf8);
+	WideCharToMultiByte(CP_UTF8, 0, err_str, size, err_str_utf8, utf8_size, NULL, NULL);
 
-	giterr_set(GITERR_NET, "%s: %s", str, err_str);
+	giterr_set(GITERR_NET, "%s: %s", str, err_str_utf8);
 	LocalFree(err_str);
+	git__free(err_str_utf8);
 }
 #else
 static void net_set_error(const char *str)