Fix computation of the merge base for git-validation

CIRRUS_BASE_SHA is a valid commit, but it might not be
a parent of the current PR, so it might not be available,
making a ${CIRRUS_BASE_SHA}..$something git commit range expressions
fail.

Compare https://github.com/cirruslabs/cirrus-ci-docs/issues/1279 .

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

.cirrus.yml

@@ -128,6 +128,7 @@ lint_task:
       apt-get update
       apt-get install -y libbtrfs-dev libsubid-dev
     test_script: |
+      [ -n "${CIRRUS_BASE_SHA}" ] && git fetch origin ${CIRRUS_BASE_SHA} # Make ${CIRRUS_BASE_SHA} resolvable for git-validation
       make TAGS=regex_precompile local-validate
       make lint
       make clean