Fix PlugDiff for Windows (#681)

Close #680

plug.vim

@@ -2406,7 +2406,7 @@ function! s:diff()
     call s:append_ul(2, origin ? 'Pending updates:' : 'Last update:')
     for [k, v] in plugs
       let range = origin ? '..origin/'.v.branch : 'HEAD@{1}..'
-      let diff = s:system_chomp('git log --graph --color=never --pretty=format:"%x01%h%x01%d%x01%s%x01%cr" '.s:shellesc(range), v.dir)
+      let diff = s:system_chomp('git log --graph --color=never '.join(map(['--pretty=format:"%x01%h%x01%d%x01%s%x01%cr"', range], 's:shellesc(v:val)')), v.dir)
       if !empty(diff)
         let ref = has_key(v, 'tag') ? (' (tag: '.v.tag.')') : has_key(v, 'commit') ? (' '.v.commit) : ''
         call append(5, extend(['', '- '.k.':'.ref], map(s:lines(diff), 's:format_git_log(v:val)')))