ci: avoid ugly "failure" in the `ci-config` job

ci: avoid ugly "failure" in the `ci-config` job

In the common case where users have _not_ pushed a `ci-config` branch to
configure which branches should be included in the GitHub workflow runs,
there is a big fat ugly annotation about a failure in the run's log:

	X Check failure on line 1 in .github

	  @github-actions github-actions / ci-config

	  .github#L1

	  Process completed with exit code 128.

The reason is that the `ci-config` job tries to clone that `ci-config`
branch, and even if it is configured to continue on error, the
annotation is displayed, and it is distracting.

Let's just handle this on the shell script level, so that the job's step
is not marked as a failure.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>