Add plugin to &rtp before running post-update hook with : prefix

Close #593

plug.vim

@@ -820,6 +820,10 @@ function! s:do(pull, force, todo)
       let type = type(spec.do)
       if type == s:TYPE.string
         if spec.do[0] == ':'
+          if !get(s:loaded, name, 0)
+            let s:loaded[name] = 1
+            call s:reorg_rtp()
+          endif
           call s:load_plugin(spec)
           try
             execute spec.do[1:]

test/regressions.vader

@@ -332,3 +332,12 @@ Execute (Cursor moved to another window during post-update hook):
   AssertEqual 'empty', getline(1)
   q!
   q
+
+**********************************************************************
+Execute (#593 Add plugin to &rtp before running post-update hook with : prefix):
+  call ReloadPlug()
+  call plug#begin()
+  Plug 'junegunn/vim-pseudocl', { 'on': 'XXX', 'do': ':let g:bar = pseudocl#complete#extract_words(''a b'')' }
+  call plug#end()
+  PlugInstall!
+  AssertEqual ['a', 'b'], g:bar