git-p4.py: support Python 2.4

git-p4.py: support Python 2.4

Python 2.4 lacks the following features:

   subprocess.check_call
   struct.pack_into

Take a cue from 460d1026 and provide an implementation of the
CalledProcessError exception.  Then replace the calls to
subproccess.check_call with calls to subprocess.call that check the return
status and raise a CalledProcessError exception if necessary.

The struct.pack_into in t/9802 can be converted into a single struct.pack
call which is available in Python 2.4.

Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>