ci: add single parent commit case on check-signed-off-by script

# See the License for the specific language governing permissions and

# limitations under the License.

# this retrieves the merge commit created by GH

parents=(`git log -n 1 --format=%p HEAD`)

if [[ "${#parents[@]}" -ne 2 ]]; then

  echo "This PR's merge commit is missing a parent!"

  exit 1

fi

if [[ "${#parents[@]}" -eq 1 ]]; then

  # CI doesn't use a merge commit

  commits=$(git show -s --format=%h ${parents})

elif [[ "${#parents[@]}" -eq 2 ]]; then

  # CI uses a merge commit, eg GH / Travis

  from="${parents[0]}"

  into="${parents[1]}"

  commits=$(git show -s --format=%h ${from}..${into})

else

  echo "Unexpected behavior, cannot verify more than 2 parent commits!"

  exit 1

fi

has_commits=false

for sha in $commits; do