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

git-submodule.sh

@@ -488,8 +488,11 @@ cmd_update()
 
 			if test -z "$nofetch"
 			then
+				# Run fetch only if $sha1 isn't present or it
+				# is not reachable from a ref.
 				(clear_local_git_env; cd "$path" &&
-					git-fetch) ||
+					((rev=$(git rev-list -n 1 $sha1 --not --all 2>/dev/null) &&
+					 test -z "$rev") || git-fetch)) ||
 				die "Unable to fetch in submodule path '$path'"
 			fi