scripts/getver.sh: allow conversion between git hash and revision

Add code allowing easy conversion between git commit ids and revisions.

Example:

$ ./scripts/getver.sh
r792
$ ./scripts/getver.sh r123
b7fc892e
$ ./scrpts/getver.sh b7fc892e
r123

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>