Inconsistent behavior of the path disambiguator

Hi.

When I clone a remote that has a branch `foo`, then `git checkout foo
--`, the path disamgiuator makes the operation fail. `git checkout
foo` without the disambiguator works. Following that, when branch
`foo` already exists, `git checkout foo --` works even with the
disambiguator that caused it to fail previously.

Here's a log demonstrating a full reproduction of the bug with git
1.7.2 on Linux:

=== LOG START ===

$ mkdir origin && cd origin && git init && echo 1 >> test.txt && git
add test.txt && git commit -am "First." && git branch foo
Initialized empty Git repository in /home/afine/lab/gitexp/origin/origin/.git/
[master (root-commit) 6042525] First.
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 test.txt
$ cd .. && git clone origin local && cd local
Cloning into local...
done.
$ git checkout foo --
fatal: invalid reference: foo
$ git checkout foo
Branch foo set up to track remote branch foo from origin.
Switched to a new branch 'foo'
$ git checkout master
Switched to branch 'master'
$ git checkout foo --
Switched to branch 'foo'

=== LOG END ===

This looks like a bug.

Thanks, D