Fix baseline commit and branch for code coverage (#5769)

## Problem

`HEAD` commit for a PR is a phantom merge commit which skews the baseline
commit for coverage reports.

See
https://github.com/neondatabase/neon/pull/5751#issuecomment-1790717867

## Summary of changes
- Use commit hash instead of `HEAD` for finding baseline commits for
code coverage
- Use the base branch for PRs or the current branch for pushes

.github/workflows/build_and_test.yml

@@ -586,10 +586,13 @@ jobs:
         id: upload-coverage-report-new
         env:
           BUCKET: neon-github-public-dev
+          # A differential coverage report is available only for PRs.
+          # (i.e. for pushes into main/release branches we have a regular coverage report)
           COMMIT_SHA: ${{ github.event.pull_request.head.sha || github.sha }}
+          BASE_SHA: ${{ github.event.pull_request.base.sha || github.sha }}
         run: |
-          BASELINE="$(git merge-base HEAD origin/main)"
           CURRENT="${COMMIT_SHA}"
+          BASELINE="$(git merge-base $BASE_SHA $CURRENT)"
 
           cp /tmp/coverage/report/lcov.info ./${CURRENT}.info