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

Signed-off-by: Almir Okato <almir.okato@espressif.com>

ci/check-signed-off-by.sh

@@ -12,17 +12,20 @@
 # 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