Merge branch 'mh/fetch-into-shallow'

"git fetch" into a shallow repository from a repository that does not know about the shallow boundary commits (e.g. a different fork from the repository the current shallow repository was cloned from) did not work correctly. * mh/fetch-into-shallow: t5500: add test for fetching with an unknown 'shallow' upload-pack: ignore 'shallow' lines with unknown obj-ids