clipboard: avoid clipboard during :global. #2809

This is equivalent to patches 7.4.396, 7.4.445 and 7.4.598.

vim-patch:7.4.396

Problem:    When 'clipboard' is "unnamed", :g/pat/d is very slow.
(Praful)
Solution:   Only set the clipboard after the last delete. (Christian
Brabandt)

1f285eb49a

vim-patch:7.4.445

Problem:    Clipboard may be cleared on startup.
Solution:   Set clip_did_set_selection to -1 during startup. (Christian
	    Brabandt)

1a19d37d90

vim-patch:7.4.598

Problem:    ":tabdo windo echo 'hi'" causes "* register not to be
changed.
	    (Salman Halim)
Solution:   Change how clip_did_set_selection is used and add
	    clipboard_needs_update and global_change_count.  (Christian
	    Brabandt)

af6a579263

Co-Author: @bfredl

src/nvim/ex_docmd.c

@@ -9380,6 +9380,8 @@ static void ex_folddo(exarg_T *eap)
 {
   linenr_T lnum;
 
+  start_global_changes();
+
   /* First set the marks for all lines closed/open. */
   for (lnum = eap->line1; lnum <= eap->line2; ++lnum)
     if (hasFolding(lnum, NULL, NULL) == (eap->cmdidx == CMD_folddoclosed))
@@ -9388,6 +9390,8 @@ static void ex_folddo(exarg_T *eap)
   /* Execute the command on the marked lines. */
   global_exe(eap->arg);
   ml_clearmarked();        /* clear rest of the marks */
+
+  end_global_changes();
 }
 
 static void ex_terminal(exarg_T *eap)