vim-patch:8.1.1134: buffer for quickfix window is reused for another file

Problem:    Buffer for quickfix window is reused for another file.
Solution:   Don't reuse the quickfx buffer. (Yegappan Lakshmanan)

39803d82db

src/nvim/buffer.c

@@ -1868,6 +1868,7 @@ bool curbuf_reusable(void)
           && curbuf->b_ffname == NULL
           && curbuf->b_nwindows <= 1
           && (curbuf->b_ml.ml_mfp == NULL || BUFEMPTY())
+          && !bt_quickfix(curbuf)
           && !curbufIsChanged());
 }