"git pull --rebase --recurse-submodules" learns to rebase the
branch in the submodules to an updated base.

* sb/pull-rebase-submodule:
  builtin/fetch cleanup: always set default value for submodule recursing
  pull: optionally rebase submodules (remote submodule changes only)
  builtin/fetch: parse recurse-submodules-default at default options parsing
  builtin/fetch: factor submodule recurse parsing out to submodule config