merge: move the filtering of ambiguous files to a dedicated function I have multiple reasons: - The body of `_update` is way too long - This adds typing which will help our tooling and brains understand this code more easily - This function will get more nested and complex in the next patch I've taken the liberty of rewrapping and typo-passing the docstring.

timestamp: make the reliable comparison more usable from outside This is going to be used with pre-computed times unlike in status.

timestamp: add type information to the module This is easy to do and helps both Pytype and developpers understand what objects they are dealing with.

rust-dirstate: make the reliable timestamp comparison more usable from outside This is going to be used with pre-computed times unlike in status.

rust-dirstate-map: use a more precise identity This is closer to the behavior of what Python does. So far, we were checking only the inode, but this might not be good enough for the v1 case.

dirstate-map: add a missing debug wait point when accessing the v2 docket fc8e37c380d3 added synchronization points to the dirstate to allow for race condition testing without actually requiring a time-based race condition to happen. This changes adds the `pre-read-file` wait point before we read the docket, since callers might ask for the parents before anything else is read, leading to the first read being done before the wait point. This removes some differences in test output which were presumed to be speed related, but weren't.