AX_DETECT_GIT_HEAD: detect git version with newer versions of git
AX_DETECT_GIT_HEAD: detect git version with newer versions of git

In particular, if ppcg is used as a submodule, then in newer versions
of git, .git may be a file containing the path of the real directory
that has the repository.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg