'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/ex_cmds.lua

@@ -2194,7 +2194,7 @@ return {
     command='substitute',
     flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, CMDWIN),
     addr_type=ADDR_LINES,
-    func='do_inc_sub',
+    func='ex_substitute',
   },
   {
     command='sNext',
@@ -3181,7 +3181,7 @@ return {
     enum='CMD_and',
     flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, CMDWIN, MODIFY),
     addr_type=ADDR_LINES,
-    func='do_sub',
+    func='ex_substitute',
   },
   {
     command='<',
@@ -3222,6 +3222,6 @@ return {
     enum='CMD_tilde',
     flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, CMDWIN, MODIFY),
     addr_type=ADDR_LINES,
-    func='do_sub',
+    func='ex_substitute',
   },
 }