Merge branch 'jl/submodule-fetch-on-demand'

Merge branch 'jl/submodule-fetch-on-demand'

* jl/submodule-fetch-on-demand:
  fetch/pull: Describe --recurse-submodule restrictions in the BUGS section
  submodule update: Don't fetch when the submodule commit is already present
  fetch/pull: Don't recurse into a submodule when commits are already present
  Submodules: Add 'on-demand' value for the 'fetchRecurseSubmodule' option
  config: teach the fetch.recurseSubmodules option the 'on-demand' value
  fetch/pull: Add the 'on-demand' value to the --recurse-submodules option
  fetch/pull: recurse into submodules when necessary

Conflicts:
	builtin/fetch.c
	submodule.c