fsmonitor: warn when fsmonitor could be used fsmonitor can significantly speed up operations on large working directories. But fsmonitor isn't enabled by default, so naive users may not realize there is a potential to make Mercurial faster. This commit introduces a warning to working directory updates when fsmonitor could be used. The following conditions must be met: * Working directory is previously empty * New working directory adds >= N files (currently 50,000) * Running on Linux or MacOS * fsmonitor not enabled * Warning not disabled via config override Because of the empty working directory restriction, most users will only see this warning during `hg clone` (assuming very few users actually do an `hg up null`). The addition of a warning may be considered a BC change. However, clone has printed warnings before. Until recently, Mercurial printed a warning with the server's certificate fingerprint when it wasn't explicitly trusted for example. The warning goes to stderr. So it shouldn't interfere with scripts parsing meaningful output. The OS restriction was on the advice of Facebook engineers, who only feel confident with watchman's stability on the supported platforms. .. feature:: Print warning when fsmonitor isn't being used on a large repository Differential Revision: https://phab.mercurial-scm.org/D894