api: add nvim_create_buf to create a new empty buffer.

Loading existing files into a buffer is non-trivial and requires a window.
Creating an unnamed emtpy buffer is trivial and safe though, thus worth a
special case.

Change nvim_buf_set_option to use aucmd_prepbuf. This is necessary
to allow some options to be set on a not yet displayed buffer, such
as 'buftype' option.

vim-patch:7.4.1858: Add BLN_NEW to enforce buflist_new creating new buffer

src/nvim/api/private/helpers.c

@@ -27,6 +27,7 @@
 #include "nvim/version.h"
 #include "nvim/lib/kvec.h"
 #include "nvim/getchar.h"
+#include "nvim/fileio.h"
 #include "nvim/ui.h"
 
 /// Helper structure for vim_to_object
@@ -1094,7 +1095,7 @@ static void set_option_value_for(char *key,
 {
   win_T *save_curwin = NULL;
   tabpage_T *save_curtab = NULL;
-  bufref_T save_curbuf =  { NULL, 0, 0 };
+  aco_save_T aco;
 
   try_start();
   switch (opt_type)
@@ -1115,9 +1116,9 @@ static void set_option_value_for(char *key,
       restore_win(save_curwin, save_curtab, true);
       break;
     case SREQ_BUF:
-      switch_buffer(&save_curbuf, (buf_T *)from);
+      aucmd_prepbuf(&aco, (buf_T *)from);
       set_option_value_err(key, numval, stringval, opt_flags, err);
-      restore_buffer(&save_curbuf);
+      aucmd_restbuf(&aco);
       break;
     case SREQ_GLOBAL:
       set_option_value_err(key, numval, stringval, opt_flags, err);

src/nvim/api/vim.c

@@ -955,6 +955,25 @@ void nvim_set_current_win(Window window, Error *err)
   }
 }
 
+/// Create new empty buffer
+///
+/// @param listed whether the buffer should be listed
+/// @param[out] err Error details, if any
+/// @return the buffer handle or 0 when error
+Buffer nvim_create_buf(Boolean listed, Error *err)
+  FUNC_API_SINCE(6)
+{
+  try_start();
+  Buffer buffer = buflist_add(NULL,
+                              BLN_NOOPT | BLN_NEW | (listed ? BLN_LISTED : 0));
+  if (!try_end(err) && buffer == 0) {
+    api_set_error(err,
+                  kErrorTypeException,
+                  "Failed to create buffer");
+  }
+  return buffer;
+}
+
 /// Gets the current list of tabpage handles.
 ///
 /// @return List of tabpage handles