Merge #20390: CI/Cirrus: Skip merge_base step for non-PRs

20e491d CI/Cirrus: Skip merge_base step for non-PRs (Luke Dashjr)

Pull request description:

  CIRRUS_BASE_BRANCH is a PR-specific variable and undocumented on non-PR builds.
  In practice (at the moment), it seems to be HEAD, which in private repositories can be pretty much anything, causing CI to fail if it can't be cleanly merged.

  By checking CIRRUS_PR first, we can reliably do CI builds of branches outside PRs.

ACKs for top commit:
  MarcoFalke:
    review ACK 20e491d

Tree-SHA512: 9fd8db2e19a3145f7dccfca107631b20df8c94d385f624e2bcef2fa18e38bf3e23c6c68fc8241decedbf1413bf69ca572cff75e1ccf82c09ac50443001ec5ae5

Author: MarcoFalke

.cirrus.yml

@@ -29,6 +29,7 @@ global_task_template: &GLOBAL_TASK_TEMPLATE
   depends_releases_cache:
     folder: "/tmp/cirrus-ci-build/releases"
   merge_base_script:
+    - if [ "$CIRRUS_PR" = "" ]; then exit 0; fi
     - bash -c "$PACKAGE_MANAGER_INSTALL git"
     - git fetch $CIRRUS_REPO_CLONE_URL $CIRRUS_BASE_BRANCH
     - git config --global user.email "[email protected]"