'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)

test/unit/buffer_spec.lua

@@ -13,7 +13,7 @@ describe('buffer functions', function()
 
   local buflist_new = function(file, flags)
     local c_file = to_cstr(file)
-    return buffer.buflist_new(c_file, c_file, 1, flags, 0)
+    return buffer.buflist_new(c_file, c_file, 1, flags)
   end
 
   local close_buffer = function(win, buf, action, abort_if_last)