[RFC] git gui doesn't call post-checkout hook on checkout or clone

When checking out or cloning via git gui, the post-checkout hook is not called. This is a bit unexpected ...

The reason is that git gui uses git read-tree with the -u option and not git checkout and git clone. I changed git read-tree to call the post-checkout hook when called with -u and it seems to solve the problem. I would make a patch for that if you want.

But is this the right way to do this? Seems like we could surprise some users of git read-tree with this change in behaviour.

(I hope i cc'ed the right people this time ;-)

Jens