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_cmds.c

@@ -4433,8 +4433,11 @@ void ex_global(exarg_T *eap)
       smsg(_("Pattern found in every line: %s"), pat);
     else
       smsg(_("Pattern not found: %s"), pat);
-  } else
+  } else {
+    start_global_changes();
     global_exe(cmd);
+    end_global_changes();
+  }
 
   ml_clearmarked();        /* clear rest of the marks */
   vim_regfree(regmatch.regprog);