scripts/getver.sh: append short git hash based on upstream commit

The short git hash suffix printed by getver.sh is taken from the
latest local commit, change this to use the hash from latest
upstream commit if available. This is considered the intended
behavior based on commit message a642a11faca87e2a7bddc1fadb54253e2fc26e84,
introducing getver.sh.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>