We're going to require usage of `mach try` some day. Let's issue a warning on hg.mo when `mach try` isn't being used.

Comment on attachment 8909469 [details] Bug 1400439 - Issue warning when isn't used https://reviewboard.mozilla.org/r/180958/#review187588 Things aren't this simple :/ `mach try syntax` doesn't produce a try_task_config.json file. Instead, what we've done is added common messaging to all `mach try` pushes (see https://hg.mozilla.org/mozilla-central/rev/a2a1b037d342). So, we can implement the detection as searching for "pushed via `mach try`". But even that isn't sufficient. To avoid false alerts on pushes from old revisions, we'll also want to verify the changeset being pushed has a2a1b037d342 as an ancestor. This can be implemented via something like: ``` MODERN_MACH_TRY_REV = 'a2a1b037d34287da3bfac7a287a55aae779541f3' if MODERN_MACH_TRY_REV in repo and repo.changelog.isancestor(MODERN_MACH_TRY_REV, ctx.node()): ... ```