clone: run post-checkout hook when checking out

The mental model for clone is that the branch is "checked
out" (and it even says this in Documentation/git-clone.txt:
"...creates and checks out an initial branch"). Therefore it
is reasonable for users to expect that any post-checkout
hook would be run.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5403-post-checkout-hook.sh

@@ -71,4 +71,16 @@ test_expect_success 'post-checkout receives the right args when not switching br
         test $old = $new -a $flag = 0
 '
 
+mkdir -p templates/hooks
+cat >templates/hooks/post-checkout <<'EOF'
+#!/bin/sh
+echo $@ > $GIT_DIR/post-checkout.args
+EOF
+chmod +x templates/hooks/post-checkout
+
+test_expect_success 'post-checkout hook is triggered by clone' '
+	git clone --template=templates . clone3 &&
+	test -f clone3/.git/post-checkout.args
+'
+
 test_done