bug#22423: git-fetch does not update checked out tree when commit hash changes

[Pjotr Prins]

I can reliably reproduce this using a recent version of GNU Guix. 

When updating the commit hash to a different commit the git-fetch
derivation *does* change (I checked in guile), but the checked out git
tree in the store does not change - it gets shared between the
commits. I am not sure why the tree gets shared, but the effect is
that the same package gets installed using the same
/gnu/store/xxx-git-checkout.

Removing the git-checkout dir and updating the Hash gives a missing
dir error (as expected when they use the same).