Merge branch 'bw/push-submodule-only'

"git submodule push" learned "--recurse-submodules=only option to
push submodules out without pushing the top-level superproject.

* bw/push-submodule-only:
  push: add option to push only submodules
  submodules: add RECURSE_SUBMODULES_ONLY value
  transport: reformat flag #defines to be more readable

diff --cc builtin/push.c
Simple merge

diff --cc submodule-config.c
Simple merge

diff --cc submodule.h
Simple merge

diff --cc transport.c
Simple merge

diff --cc transport.h
Simple merge