fails to detect git dir of a submodule during installation

Hello,

I have a project, with a PBR-enabled lib as a submodule in it. Say this is the directory tree for example.

a-project/
  py-submodule/

When I "cd py-submodule; pip install .", pip does mktemp and goes into "/tmp/pip-XXXXX-build" to build the project. PBR there fails with this exception https://github.com/openstack-dev/pbr/blob/77d9ab7d07feb38281531deeeb4399017b5735d0/pbr/packaging.py#L744-L750

I added some PDB set_trace and found out that _get_git_directory() failed, because when it does _run_shell_command(["git", "rev-parse", "--git-dir"]) the working directory is the tmp folder. And the submodule is referenced via a relative dir. See:

$ cat py-submodule/.git
gitdir: ../.git/modules/py-submodule

So the git directory detection fails.

Is this a known issue? Any idea how to fix?