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

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>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49165 3c298f89-4303-0410-b956-a3cf2f4a3e73