The 'check-whitespace' CI script exits gracefully if no base commit is
provided or if an invalid revision is provided. This is not good because
if a particular CI provides an incorrect base_commit, it would fail
successfully.

This is exactly the case with the GitLab CI. The CI is using the
"$CI_MERGE_REQUEST_TARGET_BRANCH_SHA" variable to get the base commit
SHA, but variable is only defined for _merged_ pipelines. So it is empty
for regular pipelines [1]. This should've failed the check-whitespace
job.

Let's fallback to 'CI_MERGE_REQUEST_DIFF_BASE_SHA' if
"CI_MERGE_REQUEST_TARGET_BRANCH_SHA" isn't available in GitLab CI,
similar to the previous commit. Let's also add a check for incorrect
base_commit in the 'check-whitespace.sh' script. While here, fix a small
typo too.

[1]: https://docs.gitlab.com/ee/ci/variables/predefined_variables.html#predefined-variables-for-merge-request-pipelines

Signed-off-by: Karthik Nayak <redacted>
---
 .gitlab-ci.yml         |  7 ++++++-
 ci/check-whitespace.sh | 10 ++++++++--
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index dc43fc8ba8..c5a18f655a 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml

@@ -119,7 +119,12 @@ check-whitespace:
   before_script:
     - ./ci/install-dependencies.sh
   script:
-    - ./ci/check-whitespace.sh "$CI_MERGE_REQUEST_TARGET_BRANCH_SHA"
+    - |
+      if [ -z ${CI_MERGE_REQUEST_TARGET_BRANCH_SHA} ]; then
+        ./ci/check-whitespace.sh "$CI_MERGE_REQUEST_DIFF_BASE_SHA"
+      else
+        ./ci/check-whitespace.sh "$CI_MERGE_REQUEST_TARGET_BRANCH_SHA"
+      fi