Caching of file contents creates an inconsistency in the value of the
`Content-Disposition` header, allowing files that should only be sent as
attachment to instead be returned as inline, causing them to be
evaluated and executed by the receiving client. This is due to how
gitaly and the main Rails application coordinate around evaluating etags
for content freshness. This fix addresses the issue by removing caching
from this endpoint, but does not address the underlying issue (namely
that Rails can not accurately determine the file type of the requested
content, thus can not be responsible for determining appropriate or safe
Content-Disposition.)