scripts/getver.sh: prevent asking for negative rev-parse

With the case of asking an invalid version that is too big, getver.sh
might return an invalid output in the form of HEAD~-2260475641.

This is caused by BASE_REV - GET_REV using a negative number.

Prevent this by checking if BASE_REV - GET_REV actually return 0 or a
positive number and set REV variable accordingly. With the following
change, invalid revision number will result in unknown printed instead
of the invalid HEAD~-NUMBERS output.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 9e49e0a6c4)

scripts/getver.sh

@@ -21,7 +21,7 @@ try_git() {
 	r*)
 		GET_REV="$(echo $GET_REV | tr -d 'r')"
 		BASE_REV="$(git rev-list ${REBOOT}..HEAD 2>/dev/null | wc -l | awk '{print $1}')"
-		REV="$(git rev-parse HEAD~$((BASE_REV - GET_REV)))"
+		[ $((BASE_REV - GET_REV)) -ge 0 ] && REV="$(git rev-parse HEAD~$((BASE_REV - GET_REV)))"
 		;;
 	*)
 		BRANCH="$(git rev-parse --abbrev-ref HEAD)"