fix: Handle cases when BASE SHA isn't known - push events

If the first commit is pushed to a new branch, `${{ github.event.before }}` has a default value of `0000000000000000000000000000000000000000`.

When we detect such SHA, we should try to get older `BASE` SHA from the `HEAD` branch if possible.

Inspired by:

* [How can I get the previous commit before a push or merge in GitHub Action workflow? - Answered by Peter Evans](https://stackoverflow.com/a/61861763/10221282)
* [tj-actions/changed-files GitHub Action](https://github.com/tj-actions/changed-files/blob/main/diff-sha.sh#L116-L118)

src/functions.sh

@@ -41,10 +41,16 @@ is_strict_check_on_push_demanded () {
 
 # Function that picks values of BASE and HEAD commit based on triggrring event (INPUT_TRIGGERING_EVENT)
 # It sets BASE and HEAD for external use.
+# https://github.com/tj-actions/changed-files/blob/main/diff-sha.sh#L116-L118
+# https://stackoverflow.com/a/61861763/10221282
 # $? - return value - 0 on success
 pick_base_and_head_hash () {
   case ${INPUT_TRIGGERING_EVENT-${GITHUB_EVENT_NAME}} in
     "push")
+      if [[ -z "${INPUT_PUSH_EVENT_BASE}" || "${INPUT_PUSH_EVENT_BASE}" == "0000000000000000000000000000000000000000" ]]; then
+        INPUT_PUSH_EVENT_BASE=$(git rev-list -n 1 "HEAD~1")
+      fi
+
       export BASE=${INPUT_PUSH_EVENT_BASE:-}
       export HEAD=${INPUT_PUSH_EVENT_HEAD:-}
       [[ ${UNIT_TESTS:-1} -eq 0 ]] && echo "BASE:\"${BASE}\" ; HEAD:\"${HEAD}\""

test/pick_base_and_head_hash.bats

@@ -40,6 +40,25 @@ setup () {
   # assert_equal "\"${HEAD}\"" "\"${INPUT_PUSH_EVENT_HEAD}\""
 }
 
+@test "pick_base_and_head_hash() - trigger event = push - first commit/new branch" {
+  source "${PROJECT_ROOT}/src/functions.sh"
+
+  INPUT_TRIGGERING_EVENT="push"
+
+  UNIT_TESTS=0
+  INPUT_PUSH_EVENT_BASE="0000000000000000000000000000000000000000"
+  INPUT_PUSH_EVENT_HEAD="ghijkl789012"
+
+  run pick_base_and_head_hash
+  assert_success
+  assert_output \
+"first commit on new branch detected
+BASE:\"${TRUE_BASE}\" ; HEAD:\"${INPUT_PUSH_EVENT_HEAD}\""
+  # TODO: Doesn't work, don't know why...
+  # assert_equal "\"${BASE}\"" "\"${TRUE_BASE}\""
+  # assert_equal "\"${HEAD}\"" "\"${INPUT_PUSH_EVENT_HEAD}\""
+}
+
 @test "pick_base_and_head_hash() - trigger event = pull_request" {
   source "${PROJECT_ROOT}/src/functions.sh"