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

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>