revlog: fix a bug where NULL_NODE failed to be resolved to NULL_REV The problem is that nodemap already takes care about NULL_NODE resolution (in `validate_candidate` in `nodemap.rs`), so the special handling in `rev_from_node` is unnecessary and incorrect.

tests: reproduce the nullrev bug in an end-to-end test

rust-config: fix incorrect coercion of null values to false As explained in the previous changeset: Probably being too trigger happy about boolean values, I incorrectly set the transform for a `None` to a `Some(false)`. It would cause for example the `ui.formatted` value to be set to `Some(false)`, which turns off the colors among other things, when `None` would trigger the automatic behavior.

rust-config: show default `null` is coerced incorrectly to `false` Probably being too trigger happy about boolean values, I incorrectly set the transform for a `None` to a `Some(false)`. It would cause for example the `ui.formatted` value to be set to `Some(false)`, which turns off the colors among other things, when `None` would trigger the automatic behavior. This is fixed in the next commit.

rust-config: fix fallback to default not parsing the default value When a config item's default is a string, it sometimes needs to be parsed into another type, like in the case of `cmdserver.max-log-size`, that returns a number of bytes from a human-readable amount like `25MB`. The logic for the fix is explained inline.

rust-config: demonstrate a bug when falling back to non-trivial default values The default value (when the user hasn't configured anything) is not run through the value parser, causing a programming error to happen because of type mismatch. This will be fixed in the next commit.

rust: make `Revision` a newtype This change is the one we've been building towards during this series. The aim is to make `Revision` mean more than a simple integer, holding the information that it is valid for a given revlog index. While this still allows for programmer error, since creating a revision directly and querying a different index with a "checked" revision are still possible, the friction created by the newtype will hopefully make us think twice about which type to use. Enough of the Rust ecosystem relies on the newtype pattern to be efficiently optimized away (even compiler in codegen tests¹), so I'm not worried about this being a fundamental problem. [1] https://github.com/rust-lang/rust/blob/7a70647f195f6b0a0f1ebd72b1542ba91a32f43a/tests/codegen/vec-in-place.rs#L47