'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/fileio.c

@@ -5067,7 +5067,7 @@ void buf_reload(buf_T *buf, int orig_mode)
     savebuf = NULL;
   } else {
     // Allocate a buffer without putting it in the buffer list.
-    savebuf = buflist_new(NULL, NULL, (linenr_T)1, BLN_DUMMY, 0);
+    savebuf = buflist_new(NULL, NULL, (linenr_T)1, BLN_DUMMY);
     if (savebuf != NULL && buf == curbuf) {
       /* Open the memline. */
       curbuf = savebuf;
@@ -6409,8 +6409,9 @@ win_found:
           && curbuf != aco->new_curbuf
           && buf_valid(aco->new_curbuf)
           && aco->new_curbuf->b_ml.ml_mfp != NULL) {
-        if (curwin->w_s == &curbuf->b_s)
+        if (curwin->w_s == &curbuf->b_s) {
           curwin->w_s = &aco->new_curbuf->b_s;
+        }
         --curbuf->b_nwindows;
         curbuf = aco->new_curbuf;
         curwin->w_buffer = curbuf;