update: filter the ambiguous mtime in update directly Right now, this filtering is done by `dirstate.write` using the time of `dirstate.write` method call. However that filtering is done "too late" It works "fine" as most command are "fast enough", and race rare enough. We are about to change the mtime filtering logic in the dirstate to be more accurate and reliable. However `hg update` will still need such filtering (mostly because it is actually quite racy, even with the existing filtering). So we explicitly implement a similar logic here. Before removing the older one later in the series. Differential Revision: https://phab.mercurial-scm.org/D11784

dirstate: move "get fs now" in the timestamp utility module We will need it during update. Differential Revision: https://phab.mercurial-scm.org/D11783

dirstate-item: allow mtime to be None in "parentdata" This will be useful to filter out unreliable mtime. Differential Revision: https://phab.mercurial-scm.org/D11782

dirstate: add a comment about a racy piece of code during updates This is a bit that is not really correct but works "fine" in practice. Let us write the details down so that people stop wondering how that logic might be correct… It is not. Differential Revision: https://phab.mercurial-scm.org/D11781

tests: add missing `head` for when things go wrong See comment above the changed line, you can get a millions of line of output in case of failure. When don't need to have them all. Differential Revision: https://phab.mercurial-scm.org/D11780

tests: ensure a status will have non ambiguous mtime in some race test For the test to work, we need some mtime to be recorded. For them to be recorded, they need to be "clearly in the past", otherwise edit with the same mtime would be possible. Strictly speaking there might be file system with a minimal mtime increment longer than one second. However it is unlikely that we will run the test on them for now. We can be smarter about this in the future if it becomes necessary. Differential Revision: https://phab.mercurial-scm.org/D11779

tests: make sure no ambiguities remains after the commit This will help to stabilize part of the test that are not relevant for what is actually tested. Differential Revision: https://phab.mercurial-scm.org/D11778

dirstate: clarify a `hg update` invocation in a test It is common for readers of that test to confuse the `hg co` call with a `hg commit`, while it actually means `hg checkout`, an alias for the more common `hg update. So let us use the clearer version. Differential Revision: https://phab.mercurial-scm.org/D11777

test: mark rhg output as flaky rhg is not updating the dirstate on status yet, which make this part of the test flaky. This will be fixed soon. Differential Revision: https://phab.mercurial-scm.org/D11833

chistedit: explain which order the commits are presented in It's not obvious which order the commits in chistedit (and text-based histedit), so let's add a note about it. Differential Revision: https://phab.mercurial-scm.org/D11832