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>
(cherry picked from commit 6d4e02fe5f2e79fceb6cf672f6f822714db6d0fe)
(cherry picked from commit ee1ead81891d7a0d4e62e5ba89ebee9db6359e76)
(cherry picked from commit 2dbec730e875f44a1d8a53e123fef428a14add95)
