Merge branch 'sb/rev-parse-show-superproject-root'

Merge branch 'sb/rev-parse-show-superproject-root'

From a working tree of a repository, a new option of "rev-parse"
lets you ask if the repository is used as a submodule of another
project, and where the root level of the working tree of that
project (i.e. your superproject) is.

* sb/rev-parse-show-superproject-root:
  rev-parse: add --show-superproject-working-tree