Merge branch 'hv/submodule-not-yet-pushed-fix' into maint

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