Backend: add argument to `fileDiffs` to filter diffs to a file or directory

Created by: camdencheek

This adds an argument to fileDiffs so that we can filter the resulting diffs to a set of files or directories. This should be the extent of the backend changes required to implement a view of the changes for a single file as described here

Examples:

Get change in `dev/gqltest/search_test.go` over the last 50 commits

{
  repository(name: "github.com/sourcegraph/sourcegraph") {
    name
    comparison(base:"HEAD~50", head:"HEAD") {
      fileDiffs(paths:["dev/gqltest/search_test.go"]){
        nodes{
          hunks {
            body
          }
        }
      }
    }
  }
}

Get all commits that touch the directory `internal/search/job/jobutil` and the diff associated with them

{
	repository(name:"github.com/sourcegraph/sourcegraph") {
    commit(rev:"HEAD"){
      ancestors(path:"internal/search/job/jobutil") {
        nodes {
          diff{
            fileDiffs(paths:["internal/search/job/jobutil"]){
              nodes{
                hunks {
                  body
                }
              }
            }
          }
        }
      }
    }
  }
}

Test plan

Added unit test, tested manually to make sure git stuff works as expected.