Fix autorebase error reporting

The logic used for detecting the commit breaking an autorebase does not
work correctly if the offending commit is not the first one applied
during the "reverse rebase".  Fix by using REBASE_HEAD instead of
processing the output of "git status" in a convoluted way.

Furthermore, the approach used for identifying the first offending merge
request in the case of a successful autorebase followed by a failed
build only works correctly if the base branch is not autorebased itself.
Since a solution that would work correctly for a branch autorebased on
top of a branch that only moves forward does not work correctly for a
branch autorebased on top of another autorebased branch and vice versa,
accurately identifying the most likely culprit after a successful
autorebase is a very complicated and brittle task.  Since reporting no
details at all is arguably better than reporting false details, only
produce a minimal error notification if the build fails after a
successful autorebase.

(cherry picked from commit 4c0e93108e)

.gitlab-ci.yml

@@ -2531,17 +2531,20 @@ merged-metadata:
     - if ! git push --force-with-lease origin "HEAD:${CI_COMMIT_REF_NAME}"; then touch .git-push-failed; exit 1; fi
   after_script:
     - if [ "${CI_JOB_STATUS}" = "success" ]; then exit 0; fi
-    - OLDEST_MERGE_COMMIT="$(git log --reverse --merges --pretty=%H "${CI_COMMIT_SHA}..${BASE_COMMIT}" | head -1)"
-    - read -r OLDEST_MERGE_REQUEST_PROJECT OLDEST_MERGE_REQUEST_ID < <(git log --max-count=1 "${OLDEST_MERGE_COMMIT}" | sed -nE 's|^\s*See merge request ([a-z-]+/bind9)!([0-9]+).*|\1 \2|p' | head -1)
     - |
+      REASON_DETAILS=""
       if git rebase --abort; then
           # Rebase failed; try applying recent commits from the base branch on top of the branch being rebased to determine which one introduces conflicts
           git rebase --rebase-merges "${CI_COMMIT_SHA}" "${BASE_COMMIT}" || true
-          CONFLICT_COMMIT="$(git status | sed -nE 's/^\s*(pick|merge -C) ([0-9a-f]+).*/\2/p' | head -1 | git rev-list -n 1 --stdin)"
+          CONFLICT_COMMIT="$(git rev-parse REBASE_HEAD)"
+          CONFLICT_COMMIT_AUTHOR="$(git log --max-count=1 --pretty="@%al" "${CONFLICT_COMMIT}")"
+          CONFLICT_COMMIT_MERGE="$(git log --reverse --merges --pretty="%H" "${CONFLICT_COMMIT}..${BASE_COMMIT}" | head -1)"
+          read -r CONFLICT_COMMIT_MERGE_REQUEST_PROJECT CONFLICT_COMMIT_MERGE_REQUEST_ID < <(git log --max-count=1 "${CONFLICT_COMMIT_MERGE}" | sed -n -E 's|^\s*See merge request ([a-z-]+/bind9)!([0-9]+).*|\1 \2|p' | head -1)
           REASON="merge conflict introduced by a change in the base branch"
+          REASON_DETAILS="${REASON_DETAILS}\n**First bad commit**: [${CONFLICT_COMMIT}](https://gitlab.isc.org/${CONFLICT_COMMIT_MERGE_REQUEST_PROJECT}/-/commit/${CONFLICT_COMMIT}) (authored by ${CONFLICT_COMMIT_AUTHOR})"
+          REASON_DETAILS="${REASON_DETAILS}\n**First bad merge request**: https://gitlab.isc.org/${CONFLICT_COMMIT_MERGE_REQUEST_PROJECT}/-/merge_requests/${CONFLICT_COMMIT_MERGE_REQUEST_ID}"
       else
           # Rebase did not fail; most likely, this is a build failure, or the job was canceled
-          CONFLICT_COMMIT="${OLDEST_MERGE_COMMIT}"
           if [ "${CI_JOB_STATUS}" = "failed" ]; then
               if [ -f ".git-push-failed" ]; then
                   REASON="branch was updated during rebase"
@@ -2552,12 +2555,10 @@ merged-metadata:
               REASON="job was canceled"
           fi
       fi
-      CONFLICT_COMMIT_AUTHOR="$(git log --max-count=1 --pretty="@%al" "${CONFLICT_COMMIT}")"
       MSG="#### :rotating_light: Autorebase error for branch \`${CI_COMMIT_REF_NAME}\` :rotating_light:"
       MSG="${MSG}\n**Job**: ${CI_JOB_URL}"
       MSG="${MSG}\n**Reason**: ${REASON}"
-      MSG="${MSG}\n**First bad commit**: [${CONFLICT_COMMIT}](https://gitlab.isc.org/${OLDEST_MERGE_REQUEST_PROJECT}/-/commit/${CONFLICT_COMMIT}) (authored by ${CONFLICT_COMMIT_AUTHOR})"
-      MSG="${MSG}\n**First bad merge request**: https://gitlab.isc.org/${OLDEST_MERGE_REQUEST_PROJECT}/-/merge_requests/${OLDEST_MERGE_REQUEST_ID}"
+      MSG="${MSG}${REASON_DETAILS}"
     - |
       curl -s -o /dev/null -X POST -H content-type:application/json -d '{"channel":"bind-9-team", "text": "'"${MSG}"'" }' "${MATTERMOST_WEBHOOK_URL}"