Use null-delimiting when interacting with git check-ignore and disable git octal quoting

[mjpieters]

Currently, the git check-ignore command result is parsed with using an unsupported legacy function in codecs, because git can output \OOO octal escape codes for any 'unusual' characters.

Instead of using undocumented functions, isort should simply disable the git quoting features:

• Disable core.quotePath (pass -c core.quotePath=).
• Use null delimiting instead of linefeed delimiting (pass -z, read and write filenames with \0 as the delimiter)

This would also allow isort to just use the subprocess encoding and decoding support, provided git is explicitly configured to expect and produce UTF-8 (set the LANG=C.UTF-8 environment variable).