Revert "Update base_sha to use the last commit on the current branch …

…for push event (#235)" (#237)

This reverts commit d924fcc.

entrypoint.sh

@@ -44,14 +44,13 @@ if [[ $exit_status -ne 0 ]]; then
 fi
 
 if [[ -z $GITHUB_BASE_REF ]]; then
-  TARGET_BRANCH=${GITHUB_REF/refs\/heads\//}
-  CURRENT_BRANCH=$TARGET_BRANCH
-
   if [[ -z $INPUT_BASE_SHA ]]; then
-    PREVIOUS_SHA=$(git rev-parse "$TARGET_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
+    PREVIOUS_SHA=$(git rev-parse HEAD^1 2>&1) && exit_status=$? || exit_status=$?
   else
     PREVIOUS_SHA=$INPUT_BASE_SHA
   fi
+  TARGET_BRANCH=${GITHUB_REF/refs\/heads\//}
+  CURRENT_BRANCH=$TARGET_BRANCH
 
   if [[ $exit_status -ne 0 ]]; then
     echo "::warning::Unable to determine the previous commit sha"