rust: blanket implementation of Graph for Graph references The need comes from the fact that `AncestorsIterator` and many Graph-related algorithms take ownership of the `Graph` they work with. This, in turn is due to them needing to accept the `Index` instances that are provided by the Python layers (that neither rhg nor `RHGitaly` use, of course): the fact that nowadays the Python layer holds an object that is itself implemented in Rust does not change the core problem that they cannot be tracked by the borrow checker. Even though it looks like cloning `Changelog` would be cheap, it seems hard to guarantee that on the long run. The object is already too rich for us to be comfortable with it, when using references is the most natural and guaranteed way of proceeding. The added test seems a bit superfleous, but it will act as a reminder that this feature is really useful until something in the Mercurial code base actually uses it.

pub type ExitCode = i32;

/// Successful exit
pub const OK: ExitCode = 0;

/// Generic abort
pub const ABORT: ExitCode = 255;

// Abort when there is a config related error
pub const CONFIG_ERROR_ABORT: ExitCode = 30;

/// Indicates that the operation might work if retried in a different state.
/// Examples: Unresolved merge conflicts, unfinished operations
pub const STATE_ERROR: ExitCode = 20;

// Abort when there is an error while parsing config
pub const CONFIG_PARSE_ERROR_ABORT: ExitCode = 10;

/// Generic something completed but did not succeed
pub const UNSUCCESSFUL: ExitCode = 1;

/// Command or feature not implemented by rhg
pub const UNIMPLEMENTED: ExitCode = 252;

/// The fallback path is not valid
pub const INVALID_FALLBACK: ExitCode = 253;