rhg: fix race when a fixup file is deleted on disk See next changeset for the other race in the same kind of logic and why there are two different places.

dirstate: test a `hg status` raced by a `hg remove` This shows that `rhg` is misbehaving here.

dirstate: tests racing status with both dirstate-v2 append and rewrite The way the racing process touches the dirstate results in different challenges for the raced process. We now test each variant in the `test-dirstate-status-race.t` tests.

dirstate-v2: add devel config option to control write behavior This will help us to write predictable tests checking behavior in each case.

dirstate: use more than a bool to control append behavior When writing dirstate-v2, we might either append to the existing file, or create a new file. We are about to introduce some configuration to control this behavior. As a prelude, we change the current way the behavior was automatically controlled to make the change smaller/clearer.

dirstate: cover each dirstate version when testing for status race Previously we were only testing it with the default (dirstate-v1 currently). Now we explicitly test each variant.

dirstate: test a `hg status` raced by another `hg status` This shows that `rhg` is misbehaving here.

dirstate: test a `hg status` raced by a `hg update` This shows that `rhg` is misbehaving here.

dirstate: test a `hg status` raced by a `hg commit` This shows that `rhg` is misbehaving here.

dirstate: test a `hg status` raced by a `hg add` This shows that `rhg` is misbehaving here.

dirstate: add a way to test races happening during status We add the `devel.sync.status.pre-dirstate-write-file` config option to easily test what happens when other operations happen during the window where `hg status` is done working but has not updated the cache on disk yet. We introduce the framework for testing such races too, actual tests will be added in the next changesets. For now the test is only checking dirstate-v1. We will extend the test coverage later too. Check test documentation for details. Code change from Raphaël Gomès <rgomes@octobus.net> Test change from Pierre-Yves David <pierre-yves.david@octobus.net>

testing: introduce util function to synchronize concurrent commands on files This is an extension of mechanisms that the tests have been using for a while. To be able to also control the execution in Rust, we introduce utility to perform such `wait_on_file` logic based on some configuration value. This will be used in the tests introduced in the next changesets.

dirstate: add some debug output when writing the dirstate This helps debugging Mercurial behavior in the dirstate-v2 case.

run-tests: make it possible to nest conditionals This is not that hard to implement and makes our life easier on a regular basis.

dirstate-v2: don't mmap the data file when on NFS `mmap` on NFS will trigger a SIGBUS when the mmap'ed file is deleted, which wouldn't work in our case. Also, the performance advantage of using mmap on NFS is debatable at best.