git-gui: run post-checkout hook on checkout

git-gui: run post-checkout hook on checkout

git-gui is using "git-read-tree -u" for checkout which doesn't
invoke the post-checkout hook as a plain git-checkout would.
So git-gui must call the hook itself.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>