Provide something like Git attributes

The checked-in .gitattributes file allows to control some aspects of the repository.

In particular, it is interpreted by github-linguist, the engine taking care of programming language analysis, to provide exclusion for vendored libraries and direct detection rules.

As a practical example, .t files used for cram and Mercurial tests are detected as Perl by github-linguist. There is even a dedicated github-linguist issue on the subject. Projects like Octobus' mercurial-devel and evolve could force them to something more appropriate right now. The price to pay would be that it's called .gitattributes and that it probably wouldn't be forward compatible with how native Mercurial repositories will work in the not-so-far future.

There are potentially other use cases for a shared, checked-in configuration file, but we obviously should not call it.gitattributes. Does something similar already exist in Mercurial land that we could piggy-back on ? Should we introduce a .heptapod-attributes?