Introduce "file_exists?" RPC to check for repository file existence
Summary
There are cases where we need to check for file existence in the repository at the specified path. Today we use #blobs_at method and retrieve file contents even if we don't need the context but just need file existence check. It produces additional load and sometimes fails for heavy background jobs.
Proposal
Investigate if we can make an efficient version of "any_file_exists_at?" check which does not retrieve full blob and just returns boolean value. If efficient version is possible then implement it. Multiple pathes and references would be great to have. Ideally the method accepts list of filepathes with git references and returns true if at least file exists for given references and false otherwise.