Merge branch 'jt/gitlab-ci-base-fix'

Two CI tasks, whitespace check and style check, work on the
difference from the base version and the version being checked, but
the base was computed incorrectly in GitLab CI in some cases, which
has been corrected.

* jt/gitlab-ci-base-fix:
  ci: fix base commit fallback for check-whitespace and check-style

Author: gitster

.gitlab-ci.yml

@@ -216,7 +216,7 @@ check-whitespace:
   # be defined in all pipelines.
   script:
     - |
-      R=${CI_MERGE_REQUEST_TARGET_BRANCH_SHA-${CI_MERGE_REQUEST_DIFF_BASE_SHA:?}} || exit
+      R=${CI_MERGE_REQUEST_TARGET_BRANCH_SHA:-${CI_MERGE_REQUEST_DIFF_BASE_SHA:?}} || exit
       ./ci/check-whitespace.sh "$R"
   rules:
     - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
@@ -236,7 +236,7 @@ check-style:
   # be defined in all pipelines.
   script:
     - |
-      R=${CI_MERGE_REQUEST_TARGET_BRANCH_SHA-${CI_MERGE_REQUEST_DIFF_BASE_SHA:?}} || exit
+      R=${CI_MERGE_REQUEST_TARGET_BRANCH_SHA:-${CI_MERGE_REQUEST_DIFF_BASE_SHA:?}} || exit
       ./ci/run-style-check.sh "$R"
   rules:
     - if: $CI_PIPELINE_SOURCE == 'merge_request_event'