dirstate: set identity whenever we read the dirstate's v2 docket The docket can be loaded outside of a full read (for exemple when pre-fetching parents), so the current code would read/set the identity after loading the data, opening a race condition: A0: first process docket is read B0: other process appends new data to the dirstate (and changes the docket) A1: first process sets the identity (based on pre-B content, but with post-B identity) A1: first process loads the dirstatemap from the data file A1: first process does not detect the race and overwrites the update from B.

dirstate: factor the identity setting code in the dirstate map We need it in more locations, so let us start factoring thing out first to make sure the same code is called everywhere. This bears some similarity with 85746485a4dd on default, but at a smaller scope and for a different purpose.

dirstate: simplify the dirstate's read race testing Now that most code behaves properly, we can simplify the expected matching.

dirstate: deal with read-race for pure rust code path (rhg) If we cannot read the dirstate data, this is probably because a writing process wrote it under our feet. So refresh the docket and try again a handful of time.

dirstate: deal with read-race for python code using rust object If we cannot read the dirstate data, this is probably because a writing process wrote it under our feet. So refresh the docket and try again a handful of time.

dirstate: deal with read-race for pure python code If we cannot read the dirstate data, this is probably because a writing process wrote it under our feet. So refresh the docket and try again a handful of time.

dirstate: abstract the reading of the data file in v2 in a method We will need more changes to avoid some race conditions during read, so we first isolate the simple logic before making it more complicated.

dirstate: add append/new-file variants in the dirstate's read race tests This covers more ground and finds more bugs. At that point I gave up on making things as `known-bad-output` / `missing-correct-output` as this gets too messy.

dirstate: add a synchronisation point in the middle of the read This will be useful to test some more race conditions around dirstate.

dirstate: add v1-v2 variants to the dirstate's read race tests More cases mean different issues.