Merge branch 'hv/submodule-alt-odb' into maint

When a submodule repository uses alternate object store mechanism, some commands that were started from the superproject did not notice it and failed with "No such object" errors. The subcommands of "git submodule" command that recursed into the submodule in a separate process were OK; only the ones that cheated and peeked directly into the submodule's repository from the primary process were affected. By Heiko Voigt * hv/submodule-alt-odb: teach add_submodule_odb() to look for alternates