bug: nested submodules get an absolute gitdir path

Hi all,

Starting with git v2.7.0,

submodules two levels deep get a .git contents of:
 gitdir: /absolute/path/name

This breaks repos that are moved or copied:

fatal: Not a git repository: /tmp/tmp.gIyxKWIC26/repo/.git/modules/sub/modules/sub fatal: 'git status --porcelain' failed in submodule sub

The script https://gitlab.com/snippets/17301 reproduces the issue.

git bisect points to commit ee8838d1.

thanks,