Merge branch 'mk/submodule-gitdir-path' into maint

The submodule code has been taught to work better with separate
work trees created via "git worktree add".

* mk/submodule-gitdir-path:
path: implement common_dir handling in git_pathdup_submodule()
submodule refactor: use strbuf_git_path_submodule() in add_submodule_odb()

diff --cc cache.h
Simple merge

diff --cc path.c
Simple merge

diff --cc setup.c
Simple merge