[funny] "git checkout -t origin/xyzzy" seems to misbehave

When you

 * are on a branch "foo" that is not "bar",
 * have "origin/bar",
 * and already have a local branch "bar",

"git checkout -t origin/bar" seems to misbehave.

    $ git clone -s git.junio victim-002
    $ cd victim-002
    $ git branch
    * master
    $ git checkout -t origin/next
    Branch next set up to track remote branch refs/remotes/origin/next.
    Switched to a new branch "next"
    $ git checkout -t origin/master
    fatal: A branch named 'master' already exists.
    $ git branch
      master
    * next
    $ git diff --cached --shortstat
     60 files changed, 2378 insertions(+), 3412 deletions(-)
    $ git diff --cached master
    $ exit

The first "checkout -t" is fine.  The failed one seems to have already
updated the index and the work tree when it notices that it cannot create
a new branch.

I suspect "-t" does not have to be in effect to trigger this; in other
words, "git checkout -b master origin/master" would have the same issue.

I'm reporting this before digging it further myself, because I may not be
able to diagnose this before I leave for a vacation.