'inccommand': rework

- Eliminate/isolate static/global variables
- Remove special-case parameter from buflist_new()
- Remove special-case ECMD_RESERVED_BUFNR
- To determine when u_undo_and_forget() should be done, check
  b_changedtick instead of a heuristic.
- use mb_string2cells() instead of strlen() to measure the :sub patterns
- call ml_close() before buf_clear_file(). Avoids leaks caught by ASan.

Original patch by:
  Robin Elrharbi-Fleury (Robinhola)
  Audrey Rayé (Adrey06)
  Philémon Hullot (DesbyP)
  Aymeric Collange (aym7)
  Clément Guyomard (Clement0)

src/nvim/memline.c

@@ -2340,14 +2340,13 @@ int ml_replace(linenr_T lnum, char_u *line, int copy)
   return OK;
 }
 
-/*
- * Delete line 'lnum' in the current buffer.
- *
- * Check: The caller of this function should probably also call
- * deleted_lines() after this.
- *
- * return FAIL for failure, OK otherwise
- */
+/// Delete line `lnum` in the current buffer.
+///
+/// @note The caller of this function should probably also call
+/// deleted_lines() after this.
+///
+/// @param message  Show "--No lines in buffer--" message.
+/// @return FAIL for failure, OK otherwise
 int ml_delete(linenr_T lnum, int message)
 {
   ml_flush_line(curbuf);