'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/quickfix.c

@@ -3327,7 +3327,7 @@ load_dummy_buffer (
   aco_save_T aco;
 
   // Allocate a buffer without putting it in the buffer list.
-  newbuf = buflist_new(NULL, NULL, (linenr_T)1, BLN_DUMMY, 0);
+  newbuf = buflist_new(NULL, NULL, (linenr_T)1, BLN_DUMMY);
   if (newbuf == NULL) {
     return NULL;
   }