Add test for percent character in file name

  AssertEqual 'jinja', &filetype

Given (File name with percent character):

  Hello world

Execute:

  silent execute 'file' fnameescape('foo%bar.html.jinja')

  filetype detect

Then:

  AssertEqual 'html.jinja', &filetype

  AssertEqual 'foo%bar.html.jinja', bufname()

Given (Do not double-detect jinja):

  {# This is a Jinja comment #}