feat(aucmd_win): allow crazy things with hidden buffers (#21250)

Problem:    Crash when doing crazy things with hidden buffers.
Solution:   Dynamically allocate the list of autocommand windows.

src/nvim/diff.c

@@ -2944,11 +2944,6 @@ void ex_diffgetput(exarg_T *eap)
 
     // Set curwin/curbuf to buf and save a few things.
     aucmd_prepbuf(&aco, curtab->tp_diffbuf[idx_other]);
-    if (curbuf != curtab->tp_diffbuf[idx_other]) {
-      // Could not find a window for this buffer, the rest is likely to
-      // fail.
-      goto theend;
-    }
   }
 
   const int idx_from = eap->cmdidx == CMD_diffget ? idx_other : idx_cur;