'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/mark.c

@@ -474,7 +474,7 @@ static void fname2fnum(xfmark_T *fm)
     p = path_shorten_fname(NameBuff, IObuff);
 
     // buflist_new() will call fmarks_check_names()
-    (void)buflist_new(NameBuff, p, (linenr_T)1, 0, 0);
+    (void)buflist_new(NameBuff, p, (linenr_T)1, 0);
   }
 }