'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/window.c

@@ -2903,7 +2903,7 @@ static int win_alloc_firstwin(win_T *oldwin)
   if (oldwin == NULL) {
     /* Very first window, need to create an empty buffer for it and
      * initialize from scratch. */
-    curbuf = buflist_new(NULL, NULL, 1L, BLN_LISTED, 0);
+    curbuf = buflist_new(NULL, NULL, 1L, BLN_LISTED);
     if (curbuf == NULL) {
       return FAIL;
     }