'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/syntax.c

@@ -7160,16 +7160,13 @@ int syn_namen2id(char_u *linep, int len)
  */
 int syn_check_group(char_u *pp, int len)
 {
-  int id;
-  char_u  *name;
-
-  name = vim_strnsave(pp, len);
-
-  id = syn_name2id(name);
-  if (id == 0)                          /* doesn't exist yet */
+  char_u  *name = vim_strnsave(pp, len);
+  int id = syn_name2id(name);
+  if (id == 0) {  // doesn't exist yet
     id = syn_add_group(name);
-  else
+  } else {
     xfree(name);
+  }
   return id;
 }