update_git_source_package: use 12 char long hash for bump commits

update_git_source_package: use 12 char long hash for bump commits

The default value for abbrev commit is too small and might pose invalid
hash referencing one hash with another. Use the current common size of
12 char for abbrev hash to better reference them following how it's done
on linux kernel and github.com.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>