mimalloc: make MIMALLOC_SHOW_STATS work with redirected stderr

Setting `MIMALLOC_SHOW_STATS` to ask mimalloc to print out something
after the process is done is the easiest way to verify that a
mimalloc-enabled Git is running.

So it better work and not try to write to a Win32 Console when it got a
regular file handle instead or, as is the case in Git for Windows'
regular Git Bash window, an emulated pseudo terminal.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mimalloc/options.c

@@ -179,8 +179,15 @@ static void mi_out_stderr(const char* msg, void* arg) {
   if (!_mi_preloading()) {
     // _cputs(msg);  // _cputs cannot be used at is aborts if it fails to lock the console
     static HANDLE hcon = INVALID_HANDLE_VALUE;
+    static int write_to_console;
     if (hcon == INVALID_HANDLE_VALUE) {
+      CONSOLE_SCREEN_BUFFER_INFO sbi;
       hcon = GetStdHandle(STD_ERROR_HANDLE);
+      write_to_console = GetConsoleScreenBufferInfo(hcon, &sbi) ? 1 : 0;
+    }
+    if (!write_to_console) {
+      fputs(msg, stderr);
+      return;
     }
     const size_t len = strlen(msg);
     if (hcon != INVALID_HANDLE_VALUE && len > 0 && len < UINT32_MAX) {