Documentation: document post-rewrite hook

Documentation: document post-rewrite hook

This defines the behaviour of the post-rewrite hook support, which
will be implemented in the following patches.

We deliberately do not document how often the hook will be invoked per
rewriting command, but the interface is designed to keep that at
"once".  This would currently not matter too much, since both rebase
and filter-branch are shellscripts and spawn many processes anyway.
However, when a fast sequencer in C is implemented, it will be
beneficial to only have to run the hook once.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>