Path to the Git repository directory can be overriden by using the '$GIT_DIR'
environment variable. This patch improves detection of Git repository by using
'git-rev-parse', which respects '$GIT_DIR' environment variable, instead of just
checking the existence of '.git' directory.
Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>
SVN-Revision: 49165