patch 8.1.0877: new buffer used every time the quickfix window is opened

Problem:    New buffer used every time the quickfix window is opened.
Solution:   Reuse the buffer. (Yegappan Lakshmanan, closes #3902)

src/testdir/test_quickfix.vim

@@ -3899,3 +3899,46 @@ func Test_viscol()
   set efm&
   call delete('Xfile1')
 endfunc
+
+" Test for the quickfix window buffer
+func Xqfbuf_test(cchar)
+  call s:setup_commands(a:cchar)
+
+  " Quickfix buffer should be reused across closing and opening a quickfix
+  " window
+  Xexpr "F1:10:Line10"
+  Xopen
+  let qfbnum = bufnr('')
+  Xclose
+  " Even after the quickfix window is closed, the buffer should be loaded
+  call assert_true(bufloaded(qfbnum))
+  Xopen
+  " Buffer should be reused when opening the window again
+  call assert_equal(qfbnum, bufnr(''))
+  Xclose
+
+  if a:cchar == 'l'
+    %bwipe
+    " For a location list, when both the file window and the location list
+    " window for the list are closed, then the buffer should be freed.
+    new | only
+    lexpr "F1:10:Line10"
+    let wid = win_getid()
+    lopen
+    let qfbnum = bufnr('')
+    call assert_match(qfbnum . ' %a-  "\[Location List]"', execute('ls'))
+    close
+    " When the location list window is closed, the buffer name should not
+    " change to 'Quickfix List'
+    call assert_match(qfbnum . '  h-  "\[Location List]"', execute('ls'))
+    call assert_true(bufloaded(qfbnum))
+
+    new | only
+    call assert_false(bufloaded(qfbnum))
+  endif
+endfunc
+
+func Test_qfbuf()
+  call Xqfbuf_test('c')
+  call Xqfbuf_test('l')
+endfunc