scripts: fix ABI validator when revision is a tag

scripts: fix ABI validator when revision is a tag

Commit 9cbae2aa64eb managed to break the only previously supported
case where a tag is used as a revision, due to git show output
differing between tags and other objects. The hash is on the last
line of the output in both cases though so just grab that.

Fixes: 9cbae2aa64eb ("scripts: support any git revisions as ABI validation range")

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>