url: do not allow %00 to represent NUL in URLs

There is no reason to allow %00 to terminate a string, so do not allow it.
Otherwise, we end up returning arbitrary content in the string (that which is
after the %00) which is effectively hidden from callers and can escape sanity
checks and validation, and possible be used in tandem with a security
vulnerability to introduce a payload.

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
