Add an additional binding for git_attr_value() that skips UTF-8 validation

[ghost]

This PR adds an addition binding that skips UTF-8 validation as described in #671 (comment) as AttrValue::always_bytes():

impl<'string> AttrValue<'string> {
    pub fn always_bytes(value: Option<&'string [u8]>) -> Self;
}

[alexcrichton]

Seems reasonable to me!