Merge branch 'sb/submodule-update-try-harder'

Merge branch 'sb/submodule-update-try-harder'

"git submodule update" attempts two different kinds of "git fetch"
against the upstream repository to grab a commit bound at the
submodule's path, but it incorrectly gave up if the first kind
(i.e. a normal fetch) failed, making the second "last resort" one
(i.e. fetching an exact commit object by object name) ineffective.
This has been corrected.

* sb/submodule-update-try-harder:
  git-submodule.sh: try harder to fetch a submodule