Use UTF-8 code page when using native ANSI sequence processing (git-for-windows#4968)

In git-for-windows#4700, I introduced a change in Git for Windows' behavior where it
would favor recent Windows 10 versions' native ANSI sequence processing
to [Git for Windows' home-grown
one](https://github.com/git-for-windows/git/blob/v2.45.1.windows.1/compat/winansi.c#L362-L439).

What I missed was that the home-grown processing _also_ ensured that
text written to the Win32 Console was carefully converted from UTF-8 to
UTF-16 encoding, while the native ANSI sequence processing would respect
the currently-set code page.

However, Git for Windows does not use the current code page at all,
always using UTF-8 encoded text internally. So let's make sure that the
code page is `CP_UTF8` when Git for Windows uses the native ANSI
sequence processing.

This fixes git-for-windows#4851.

Author: dscho

compat/winansi.c

@@ -595,11 +595,15 @@ static void detect_msys_tty(int fd)
 #endif
 
 static HANDLE std_console_handle;
-static DWORD std_console_mode;
+static DWORD std_console_mode = ENABLE_VIRTUAL_TERMINAL_PROCESSING;
+static UINT std_console_code_page = CP_UTF8;
 
-static void reset_std_console_mode(void)
+static void reset_std_console(void)
 {
-	SetConsoleMode(std_console_handle, std_console_mode);
+	if (std_console_mode != ENABLE_VIRTUAL_TERMINAL_PROCESSING)
+		SetConsoleMode(std_console_handle, std_console_mode);
+	if (std_console_code_page != CP_UTF8)
+		SetConsoleOutputCP(std_console_code_page);
 }
 
 static int enable_virtual_processing(void)
@@ -613,6 +617,14 @@ static int enable_virtual_processing(void)
 		return 0;
 	}
 
+	std_console_code_page = GetConsoleOutputCP();
+	if (std_console_code_page != CP_UTF8)
+		SetConsoleOutputCP(CP_UTF8);
+	if (!std_console_code_page)
+		std_console_code_page = CP_UTF8;
+
+	atexit(reset_std_console);
+
 	if (std_console_mode & ENABLE_VIRTUAL_TERMINAL_PROCESSING)
 		return 1;
 
@@ -622,7 +634,6 @@ static int enable_virtual_processing(void)
 			    ENABLE_VIRTUAL_TERMINAL_PROCESSING))
 		return 0;
 
-	atexit(reset_std_console_mode);
 	return 1;
 }