'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/globals.h

@@ -603,7 +603,6 @@ EXTERN int redraw_tabline INIT(= FALSE);           /* need to redraw tabline */
 EXTERN buf_T    *firstbuf INIT(= NULL);  // first buffer
 EXTERN buf_T    *lastbuf INIT(= NULL);   // last buffer
 EXTERN buf_T    *curbuf INIT(= NULL);    // currently active buffer
-EXTERN buf_T    *livebuf INIT(= NULL);   // buffer used for live actions
 
 // Iterates over all buffers in the buffer list.
 # define FOR_ALL_BUFFERS(buf) for (buf_T *buf = firstbuf; buf != NULL; buf = buf->b_next)