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

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

The marshalled dict in the response given on STDOUT by p4 uses `str` for
keys and string values. When run using python3, these values are
deserialized as `bytes`, leading to a whole host of problems as the rest
of the code assumes `str` is used throughout.

This patch changes the deserialization behaviour such that, as much as
possible, text output from p4 is decoded to native unicode strings.
Exceptions are made for the field `data` as it is usually arbitrary
binary data. `depotFile[0-9]*`, `path`, and `clientFile` are also exempt
as they contain path strings not encoded with UTF-8, and must survive
round-trip back to p4.

Conversely, text data being piped to p4 must always be encoded when
running under python3.

encode_text_stream() and decode_text_stream() were added to make these
transformations more convenient.

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