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.

rust-cpython: don't swallow the dirstate error message In case we do get a dirstate error, we want to get the full error message and not just an opaque `Dirstate error`.

dirstate-v2: check that root nodes are at the root before writing More explanations in the previous changeset.

dirstate-v2: add check of parent/child nodes being related when writing This stems from a corruption seen in a private repository. We're not sure of the source of the corruption, and it's very possible that we're seeing compounded effects of multiple writes on a corrupted dirstate. Adding this check is not expensive in itself and large writes of the dirstate are not common. This change does not catch this problem at the root node, the next one will.

admin-verify: expect a number of errors to be returned It's the responsibility of the check to handle errors, we only care about the total count to sum up the check's work. We use `admin::verify -c dirstate` to test this path at least somewhat.

admin-verify: fix error message handling `dirstate.verify` used to return tuples but does not anymore, it returns the pre-formatted error message, which is a nicer interface anyway.

admin-verify: pass p1 down to the dirstate function This was forgotten and can break with certain kinds of corruption.

Backed out changeset 3e0f86f09f26

Backed out changeset fc317bd5b637

re2: make errors quiet By default, the re2 library will output error on its own instead of keeping the error in an exception. This make re2 printing spurious error before fallback to the stdlib remodule that may accept the pattern or also fails to parse it and raise a proper error that will be handled by Mercurial. So we also pass an Option object that changes this default.