Merge branch 'hv/submodule-recurse-push'

Merge branch 'hv/submodule-recurse-push'

"git push --recurse-submodules" learns to optionally look into the
histories of submodules bound to the superproject and push them out.

By Heiko Voigt
* hv/submodule-recurse-push:
  push: teach --recurse-submodules the on-demand option
  Refactor submodule push check to use string list instead of integer
  Teach revision walking machinery to walk multiple times sequencially