Merge branch 'sb/submodule-fetch-nontip'

Merge branch 'sb/submodule-fetch-nontip'

When "git submodule update" did not result in fetching the commit
object in the submodule that is referenced by the superproject, the
command learned to retry another fetch, specifically asking for
that commit that may not be connected to the refs it usually
fetches.

* sb/submodule-fetch-nontip:
  submodule: try harder to fetch needed sha1 by direct fetching sha1