receive-pack: purge temporary data if no command is ready to run

receive-pack: purge temporary data if no command is ready to run

When pushing a hidden ref, e.g.:

    $ git push origin HEAD:refs/hidden/foo

"receive-pack" will reject our request with an error message like this:

    ! [remote rejected] HEAD -> refs/hidden/foo (deny updating a hidden ref)

The remote side ("git-receive-pack") will not create the hidden ref as
expected, but the pack file sent by "git-send-pack" is left inside the
remote repository. I.e. the quarantine directory is not purged as it
should be.

Add a checkpoint before calling "tmp_objdir_migrate()" and after calling
the "pre-receive" hook to purge that temporary data in the quarantine
area when there is no command ready to run.

The reason we do not add the checkpoint before the "pre-receive" hook,
but after it, is that the "pre-receive" hook is called with a switch-off
"skip_broken" flag, and all commands, even broken ones, should be fed
by calling "feed_receive_hook()".

Add a new test case in t5516 as well.

Helped-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Helped-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Chen Bojun <bojun.cbj@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>