Recognise the `linguist-documentation` and `linguist-detectable`
attributes in `.gitattributes` files, and use them in
`GetLanguageStats()` to make a decision whether to include a particular
file in the stats or not.
This allows one more control over which files in their repositories
contribute toward the language statistics, so that for a project that is
mostly documentation, the language stats can reflect that.
Fixes#1672.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
