vim-patch:9.1.0049: Make "[Command Line]" a special buffer name

Problem:  E95 is possible if a buffer called "[Command Line]" already
          exists when opening the cmdwin. This can also happen if the
          cmdwin's buffer could not be deleted when closing.

Solution: Un-name the cmdwin buffer, and give it a special name instead,
          similar to what's done for quickfix buffers and for unnamed
          prompt and scratch buffers. As a result, BufFilePre/Post are
          no longer fired when opening the cmdwin. Add a "command" key
          to the dictionary returned by getbufinfo() to differentiate
          the cmdwin buffer instead. (Sean Dewar)

Cherry-pick test_normal changes from v9.0.0954.

1fb4103206

src/nvim/eval.lua

@@ -3219,6 +3219,8 @@ M.funcs = {
       	bufnr		Buffer number.
       	changed		TRUE if the buffer is modified.
       	changedtick	Number of changes made to the buffer.
+      	command		TRUE if the buffer belongs to the
+      			command-line window |cmdwin|.
       	hidden		TRUE if the buffer is hidden.
       	lastused	Timestamp in seconds, like
       			|localtime()|, when the buffer was