Ignore binary files in code search to prevent Error 500

Leaving out the -I option in "git grep" would cause an Error 500 because the
resulting line would include "Binary file X matches", which would cause the project search results to crash with the following error:

```
ActionView::Template::Error (wrong argument type nil (expected Regexp)):
    1: - blob = @project.repository.parse_search_result(blob)
    2: .blob-result
    3:   .file-holder
    4:     .file-title
  app/models/repository.rb:742:in `sub'
  app/models/repository.rb:742:in `block in parse_search_result_from_grep'
  app/models/repository.rb:741:in `each_line'
  app/models/repository.rb:741:in `parse_search_result_from_grep'
  app/models/repository.rb:682:in `parse_search_result'
  app/views/search/results/_blob.html.haml:1:in `_app_views_search_results__blob_html_haml__1959871337755590783_162450160'
  app/views/search/_results.html.haml:20:in `_app_views_search__results_html_haml__2198486911700532411_58329920'
  app/views/search/show.html.haml:7:in `_app_views_search_show_html_haml___1698304427272645201_62235300'
```

See merge request !2565