Merge branch 'mh/fetch-into-shallow'

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