Investigate preloading of files on frontend

while working a customer issue that "rails" makes requests to Gitaly that will never succeed, and generate noise in the error logs. Via @wchandler:

I think rails eagerly tries to load a few files like README.md and .gitlab-ci.yml without checking if they exist, so these errors are pretty common

If this is true, we should check for the files existing before attempting the request to Gitaly.