Add bindings for git_attr_value()

[ghost]

New public APIs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AttrValue<'string> {
    True,
    False,
    String(&'string str),
    Bytes(&'string [u8]),
    Unspecified,
}

impl<'string> AttrValue<'string> {
    pub fn from_string(_: Option<&'string str>) -> Self;
    pub fn from_bytes(_: Option<&'string [u8]>) -> Self;
}

Summary

• Adds some new public APIs as described above.
• States that the return types of Repository::get_attr and Repository::get_attr_bytes will probably be changed in the next major version in their documentations.

Wrapping the return value of Repository::get_attr(_bytes) in a new type is probably better, but that's a breaking change, so it's probably not desirable unless it's ready to bump to 0.14.

Resolves #670.

[alexcrichton]

Instead of having two enums, could there perhaps be one enum with a string/bytes variant where the bytes are guaranteed to always be invalid utf-8?

[ghost]

where the bytes are guaranteed to always be invalid utf-8?

Does that mean AttrValue::new() will always perform UTF-8 validation? Could I add another new that skips UTF-8 validation and always return bytes if the value is string?

[alexcrichton]

Seems reasonable to me!

[alexcrichton]

Thanks!