Add original filename to git blame GraphQL response

Created by: olafurpg

Previously, the GraphQL endpoint for git blame didn't include the name of the original file where the blame got added. If the file got renamed, it was not possible to show the file at the commit where that blame got added. This commit adds a new filename: String! field to the Hunk type in the GraphQL schema so that you can now jump from the blame decoration to the file at the commit where the blame got added.