The code in "git push" to compute if any commit being pushed in the
superproject binds a commit in a submodule that hasn't been pushed
out was overly inefficient, making it unusable even for a small
project that does not have any submodule but have a reasonable
number of refs.

* hv/submodule-not-yet-pushed-fix:
  submodule_needs_pushing(): explain the behaviour when we cannot answer
  batch check whether submodule needs pushing into one call
  serialize collection of refs that contain submodule changes
  serialize collection of changed submodules