"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