Fix vim72 test failed

plug.vim

@@ -601,16 +601,14 @@ function! s:update(force, names)
 endfunction
 
 function! s:is_exec_helptags(docdir)
-  let result = v:false
   let exts = uniq(sort(map(s:glob(a:docdir, '*.{txt,??x}'), 'v:val[-3:]')))
   for ext in exts
     let tagname = 'tags' . (ext == 'txt' ? '' : '-' . ext[:1])
     if !filereadable(a:docdir .'/'. tagname) || empty(s:system('cd ' . s:shellesc(a:docdir) . ' && git ls-files ' . tagname))
-      let result = v:true
-      break
+      return 1
     endif
   endfor
-  return result
+  return 0
 endfunc
 
 function! plug#helptags()