git-p4: encode/decode communication with git for python3

git-p4: encode/decode communication with git for python3

Under python3, calls to write() on the stream to `git fast-import` must
be encoded.  This patch wraps the IO object such that this encoding is
done transparently.

Conversely, any text data read from subprocesses must also be decoded
before running through the rest of the pipeline.

Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com>
Reviewed-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>