ci: checkpatch scans the entire MR

Summary

• ci: checkpatch scans the entire MR

NO_DOC=internal NO_TEST=internal NO_CHANGELOG=internal

Close #28 (closed) Docs follow-up: not necessary / new issue

.gitlab-ci.yml

@@ -150,8 +150,8 @@ checkpatch:
     tags: [shell_p_t]
     script:
       - git clone --depth 1 git@github.com:tarantool/checkpatch.git
-      - git fetch origin $CI_DEFAULT_BRANCH
-      - COUNT_COMMIT=$(git log --pretty=oneline HEAD...origin/$CI_DEFAULT_BRANCH | wc -l)
+      - git fetch origin $CI_MERGE_REQUEST_TARGET_BRANCH_SHA
+      - COUNT_COMMIT=$(git log --pretty=oneline HEAD...origin/$CI_MERGE_REQUEST_TARGET_BRANCH_SHA | wc -l)
       - checkpatch/checkpatch.pl --color=always --git HEAD~${COUNT_COMMIT}..HEAD
 
 # run long tests and coverage on release build