scripts/getver.sh: Use 'git-rev-parse' to detect if tree lies in Git repository

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