git-p4.py: support Python 2.5

git-p4.py: support Python 2.5

Python 2.5 and older do not accept None as the first argument to
translate() and complain with:

   TypeError: expected a character buffer object

As suggested by Pete Wyckoff, let's just replace the call to translate()
with a regex search which should be more clear and more portable.

This allows git-p4 to be used with Python 2.5.

Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>