Support DiffPositionType of "file"

What does this MR do and why?

Support DiffPositionType of "file"

Gitlab::Diff::Position.position_type can be "file", so this needs to support the "file" position type.

Fixes #420666 (closed)

How to set up and validate locally

Sample project: https://gitlab.com/c3pmark/graphql-diff-position

Run in Graphiql:

{
  project(fullPath: "c3pmark/graphql-diff-position") {
    mergeRequest(iid: "1") {
      discussions {
        nodes {
          notes(first: 1000) {
            nodes {
              position {
                diffRefs {
                  baseSha
                  headSha
                  startSha
                }
                filePath
                positionType
                oldLine
                oldPath
                newLine
                newPath
              }
              url
              resolvable
              resolved
              userPermissions {
                resolveNote
                adminNote
              }
            }
          }
        }
      }
    }
  }
}

MR acceptance checklist

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