refactor run_receive_hook()

refactor run_receive_hook()

Running a hook has to make complex set-up to establish web of
communication between child process and multiplexer, which is common
regardless of what kind of data is fed to the hook. Refactor the parts
that is specific to the data fed to the particular set of hooks from the
part that runs the hook, so that the code can be reused to drive hooks
that take different kind of data.

Signed-off-by: Junio C Hamano <gitster@pobox.com>