Undercover uses merged results branch

= Because the coverage data was generated using the merged results branch,
checking undercoverage should also use the same branch.

- Fix git depth to fix changeset.rb:77:in `merge_base': object not found -
no match for id (Rugged::OdbError)

.gitlab/ci/rails.gitlab-ci.yml

@@ -606,17 +606,12 @@ rspec:undercoverage:
   extends:
     - .coverage-base
     - .rails:rules:rspec-undercoverage
+  variables:
+    GIT_DEPTH: 200
   stage: post-test
   needs: ["rspec:coverage"]
   script:
-    - if [ -n "$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA" ]; then
-        echo "Checking out \$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA ($CI_MERGE_REQUEST_SOURCE_BRANCH_SHA) instead of \$CI_COMMIT_SHA (merge result commit $CI_COMMIT_SHA) so we can use $CI_MERGE_REQUEST_DIFF_BASE_SHA for undercoverage in this merged result pipeline";
-        git checkout -f ${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA};
-        bundle_install_script;
-      else
-        echo "Using \$CI_COMMIT_SHA ($CI_COMMIT_SHA) for this non-merge result pipeline.";
-      fi;
-    - UNDERCOVERAGE_COMPARE="${CI_MERGE_REQUEST_DIFF_BASE_SHA:-$(git merge-base origin/master HEAD)}"
+    - UNDERCOVERAGE_COMPARE="${CI_MERGE_REQUEST_TARGET_BRANCH_SHA:-$CI_MERGE_REQUEST_DIFF_BASE_SHA}"
     - echo "Undercoverage comparing with ${UNDERCOVERAGE_COMPARE}"
     - if [ -f scripts/undercoverage ]; then
         run_timed_command "scripts/undercoverage ${UNDERCOVERAGE_COMPARE}";