reST: linguist-language override is ignored

[pjeby]

This is similar to #578, but with two key differences:

• The language that doesn't work is restructuredText, not markdown
• The files have a .txt extension, rather than no extension

Despite adding a .gitattributes of:

*.txt linguist-language=reStructuredText

The .txt files are rendered as syntax-highlighted plain text, rather than processed as markup. Ideally, markup would respect the .gitattributes, as I'm trying to migrate some older projects to Github and I don't want to pollute the commit history and disturb the codebase (the .txt files are used to generate tests and docs, so renaming them would require possibly-breaking code changes).

A sample repo with the issue can be seen here -- the .gitattributes should make this .txt file reST, but it is rendered as plain text, syntax-highlighted as reST, rather than as reST markup.