Merge branch 'hv/fetch-moved-submodules-on-demand'

Merge branch 'hv/fetch-moved-submodules-on-demand'

"git fetch --recurse-submodules" now knows that submodules can be
moved around in the superproject in addition to getting updated,
and finds the ones that need to be fetched accordingly.

* hv/fetch-moved-submodules-on-demand:
  submodule: simplify decision tree whether to or not to fetch
  implement fetching of moved submodules
  fetch: add test to make sure we stay backwards compatible