rust-revlog: add invalidation detection to `NodeTree` class
This will be useful for callers, such as `scmutil` who reuse a
`NodeTree` instance as a cache. They would otherwise get hard
errors if any mutation of the index occurred since instantiation.
This is something the C index does not provide.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true