Use the variable `$CI_MERGE_REQUEST_DIFF_BASE_SHA` instead of
`$CI_MERGE_REQUEST_TARGET_BRANCH_SHA` as it is only available in pipelines for
merged results and not available for draft MRs either.

Closes gitlab-org/labkit#34

fix: use MR diff SHA in commitlint job

Closes #34

See merge request gitlab-org/labkit!124

The default behaviour of labkit logging is to use the local timezone
for log messages. However, gitlab-shell wants to use UTC in all cases.

Add a WithTimezone LoggerOption to allow configuring this behaviour.

Add log.WithTimezone()

See merge request gitlab-org/labkit!127