receive-pack: don't pass non-existent refs to post-{receive,update} hooks

When a push specifies deletion of non-existent refs, the post post-receive and
post-update hooks receive them as input/arguments.

For instance, for the following push, where refs/heads/nonexistent is a ref
which does not exist on the remote side:

git push origin :refs/heads/nonexistent

the post-receive hook receives from standard input:

<null-sha1> SP <null-sha1> SP refs/heads/nonexistent

and the post-update hook receives as arguments:

refs/heads/nonexistent

which does not make sense since it is a no-op.

Teach receive-pack not to pass non-existent refs to the post-receive and
post-update hooks. If the push only attempts to delete non-existent refs,
these hooks are not even called.

The update and pre-receive hooks are still notified about attempted
deletion of non-existent refs to give them a chance to inspect the
situation and act on it.

[jc: mild fix-ups to avoid introducing an extra list; also added fixes to
some tests]

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>