Update secret detection template to be more robust

Changelog: changed

lib/gitlab/ci/templates/Jobs/Secret-Detection.gitlab-ci.yml

@@ -32,15 +32,22 @@ secret_detection:
     - if [ -n "$CI_COMMIT_TAG" ]; then echo "Skipping Secret Detection for tags. No code changes have occurred."; exit 0; fi
     - if [ "$CI_COMMIT_BRANCH" = "$CI_DEFAULT_BRANCH" ]; then echo "Running Secret Detection on default branch."; /analyzer run; exit 0; fi
     - |
-      git fetch origin $CI_DEFAULT_BRANCH $CI_COMMIT_REF_NAME
-      git log --left-right --cherry-pick --pretty=format:"%H" refs/remotes/origin/${CI_DEFAULT_BRANCH}..refs/remotes/origin/${CI_COMMIT_REF_NAME} >${CI_COMMIT_SHA}_commit_list.txt
-      if [[ $(wc -l <${CI_COMMIT_SHA}_commit_list.txt) -eq "0" ]]; then
-        # if git log produces 0 or 1 commits we should scan $CI_COMMIT_SHA only
-        export SECRET_DETECTION_COMMITS=$CI_COMMIT_SHA
-      else
-        # +1 because busybox wc only counts \n and there is no trailing \n
-        echo "scanning $(($(wc -l <${CI_COMMIT_SHA}_commit_list.txt) + 1)) commits"
-        export SECRET_DETECTION_COMMITS_FILE=${CI_COMMIT_SHA}_commit_list.txt
-      fi
+      # we don't need the whole history when excluding in the next `git fetch` line,
+      # so git depth=1
+      git fetch origin --depth=1 $CI_DEFAULT_BRANCH
+      # shallow clone $CI_COMMIT_REF_NAME to get commits associated with MR or push
+      git fetch --shallow-exclude=${CI_DEFAULT_BRANCH} origin $CI_COMMIT_REF_NAME
+      # determine what commits we need to scan using "git log A..B"
+      git log --no-merges --pretty=format:"%H" refs/remotes/origin/${CI_DEFAULT_BRANCH}..refs/remotes/origin/${CI_COMMIT_REF_NAME} >${CI_COMMIT_SHA}_commit_list.txt
+
+      # we need to extend the git fetch depth to the number of commits + 2 for the following reasons:
+      # because busybox wc only counts \n and there is no trailing \n (+1)
+      # include the parent commit of the base commit in this MR/Push event. This is needed because
+      # `git diff -p` needs something to compare changes in that commit against (+1)
+      git fetch --depth=$(($(wc -l <${CI_COMMIT_SHA}_commit_list.txt) + 2)) origin $CI_COMMIT_REF_NAME
+
+      # +1 because busybox wc only counts \n and there is no trailing \n
+      echo "scanning $(($(wc -l <${CI_COMMIT_SHA}_commit_list.txt) + 1)) commits"
+      export SECRET_DETECTION_COMMITS_FILE=${CI_COMMIT_SHA}_commit_list.txt
     - /analyzer run
     - rm "$CI_COMMIT_SHA"_commit_list.txt