scripts: getver.sh: append Git short hash to revision

Change getver.sh to append a short Git commit hash to the end of the artifical
revision number. This way we still have order- and comparable commit numbers
but also a direct relation to the Git commit.

The new output format will look like "r2400+2-882472e" for dirty trees or like
"r2402-882472e" for clean ones.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>