PlugDiff should be able to find pending updates

# We need the name of the default branch of origin
  git checkout some-tag-or-commit
  git log ..origin/master

plug.vim

@@ -2588,6 +2588,18 @@ function! s:append_ul(lnum, text)
   call append(a:lnum, ['', a:text, repeat('-', len(a:text))])
 endfunction
 
+function! s:git_origin_branch(spec)
+  if len(a:spec.branch)
+    return a:spec.branch
+  endif
+
+  let origin_head = a:spec.dir.'/.git/refs/remotes/origin/HEAD'
+  if !filereadable(origin_head)
+    return ''
+  endif
+  return split(readfile(origin_head)[0], '/')[-1]
+endfunction
+
 function! s:diff()
   call s:prepare()
   call append(0, ['Collecting changes ...', ''])
@@ -2602,7 +2614,9 @@ function! s:diff()
     endif
     call s:append_ul(2, origin ? 'Pending updates:' : 'Last update:')
     for [k, v] in plugs
-      let range = origin ? '..origin/'.v.branch : 'HEAD@{1}..'
+      let branch = s:git_origin_branch(v)
+      if len(branch)
+        let range = origin ? '..origin/'.branch : 'HEAD@{1}..'
         let cmd = ['git', 'log', '--graph', '--color=never']
         if s:git_version_requirement(2, 10, 0)
           call add(cmd, '--no-show-signature')
@@ -2617,6 +2631,7 @@ function! s:diff()
           call append(5, extend(['', '- '.k.':'.ref], map(s:lines(diff), 's:format_git_log(v:val)')))
           let cnts[origin] += 1
         endif
+      endif
       let bar .= '='
       call s:progress_bar(2, bar, len(total))
       normal! 2G