git-p4: convert path to unicode before processing them

git-p4: convert path to unicode before processing them

P4 allows essentially arbitrary encoding for path data while we would
perfer to be dealing only with unicode strings.  Since path data need to
survive round-trip back to p4, this patch implements the general policy
that we store path data as-is, but decode them to unicode before doing
any non-trivial processing.

A new `decode_path()` method is provided that generally does the correct
conversion, taking into account `git-p4.pathEncoding` configuration.

For python2.7, path strings will be left as-is if it only contains ASCII
characters.

For python3, decoding is always done so that we have str objects.

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>