Disable credential.helper for git fetch (#1046)

plug.vim

@@ -1532,7 +1532,7 @@ while 1 " Without TCO, Vim stack is bound to explode
     let [error, _] = s:git_validate(spec, 0)
     if empty(error)
       if pull
-        let cmd = ['git', 'fetch']
+        let cmd = s:git_version_requirement(2) ? ['git', '-c', 'credential.helper=', 'fetch'] : ['git', 'fetch']
         if has_tag && !empty(globpath(spec.dir, '.git/shallow'))
           call extend(cmd, ['--depth', '99999999'])
         endif