bug#20428: git-fetch does not always validate hash

[Ricardo Wurmus <rekado@elephly.net>]

I'm currently playing with the elogind package recipe and I'm
occasionally updating my clone of the elogind git repository.  Whenever
I do I update the value of "commit" in the package definition:

    (define-public elogind
      (let ((commit "18ee7abc9a"))
        (package
          (name "elogind")
          (version (string-append "219." commit))
          (source (origin
                    (method git-fetch)
                    (uri (git-reference
                          (url "http://git.elephly.net/software/elogind.git")
                          (commit commit)))
                    (sha256
                     (base32
                      "0lg8jgp9rl3wf9w2xfip87nx9zpjhm4js7x1z05744xiyfmvawp5"))))
          ;; ...
          (license license:lgpl2.1+))))

Upon rebuilding the package from a new commit I would expect the build
to fail with a hash validation error as I have not updated the hash yet.
However, the build procedure just continues.  I noticed that the git
checkout is still the very same as before I updated the value of
"commit".  I cannot seem to reliably force a new git checkout.