Watch files, record updates, trigger changes, etc. https://github.com/facebook/watchman
Would like to see an ebuild for this as well.
This would especially useful with mercurial, which can get a real boost on big repositories thanks to this package.
The package is already in Portage as dev-util/watchman. I think this bug report can be safely closed.
indeed, it was added few days ago
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=d428c50a040b5ae00ec1d0fc38769565108c5d24