dirstate: warn about non-explicitly rolledback parent-change Now that the invalidation is dealt with by the context manager itself, we should no longer need this in the unlocking code. We start with issuing a warning if this is the case before actually dropping this code later.

dirstate: invalidate changes when parent-change fails When an error occurs during changing parents, we should invalidate all dirstate modifications and reload the dirstate. This is currently done by a `unlock` callback on the `wlock`. To fix this anomaly, we start dealing with the error directly in the context manager and its potential nesting. The "hard" part is to make sure that, when the parent-change context are nested, we and higher level nesting do not continue to use the invalidated dirstate. We introduce dedicated code to enforce that.

setup: Ensure target directory exists with building rust extension When the rust extension is the first to be build, the target directory may not exist so the copy fails.

filemerge: add union-other-first as internal merge tool See inline documentation for details.