how to update origin/<branch> after a push to origin <branch>?

I'm stumped on this one.

I'm in a repo and I push the master branch.  If I do

  git diff --name-only origin/master..master

after the push, I still see the names of the files I pushed because I haven't updated origin/master yet, with a fetch. Of course, a "git pull" would do it, but I don't want to do that, because I might be using something other than `origin' and I don't want anything but the single branch.

So, I naively thought that

  git fetch origin master

would do the trick, but after that command the git diff above still shows the same files.

To make matters worse (!!), I made this attempt:

  $ git fetch origin master:origin/master
  From git:/repo/git/acl
   * [new branch]      master     -> origin/master
  $ git diff origin/master..master
  warning: refname 'origin/master' is ambiguous.
  $ 

Uh oh.

  $ git branch -a | grep origin/master
    origin/master
    origin/master
  $ find .git/refs -type f | grep origin
  .git/refs/heads/origin/master
  .git/refs/remotes/origin/acl81
  .git/refs/remotes/origin/HEAD
  .git/refs/remotes/origin/master
  $ cat .git/refs/heads/origin/master
  28f0f4f2cdf37640b94cdbd65406898222593f28
  $ cat .git/refs/remotes/origin/master
  4be7cc89d335421a93e4551b40bd06108a030221
  $ 

I'm going to leave my repo as is until a professionals helps me out, lest I dig the hole deeper.

Thanks.

Kevin