clone: --dissociate option to mark that reference is only temporary

While use of the --reference option to borrow objects from an
existing local repository of the same project is an effective way to
reduce traffic when cloning a project over the network, it makes the
resulting "borrowing" repository dependent on the "borrowed"
repository.  After running

	git clone --reference=P $URL Q

the resulting repository Q will be broken if the borrowed repository
P disappears.

The way to allow the borrowed repository to be removed is to repack
the borrowing repository (i.e. run "git repack -a -d" in Q); while
power users may know it very well, it is not easily discoverable.

Teach a new "--dissociate" option to "git clone" to run this
repacking for the user.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>