Fix detectition of kernel git repository in setlocalversion script [take #2]

setlocalversion script was testing the presence of .git directory in
order to find out if git is used as SCM to track the current kernel
project. However in some cases, .git is not a directory but can be a
file: when the kernel is a git submodule part of a git super project for
example.

This patch just fixes this by using 'git rev-parse --show-cdup' to check
that the current directory is the kernel git topdir. This has the
advantage to not test and rely on git internal infrastructure directly.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>