ci: cache get_base_commit function calls

The commit shall not be updated during a CI run, and this calls git
fetch, which is considerably slow at times. This reduces the git fetch
calls down to 1 from 3 in the generate-build-jobs step.

Signed-off-by: Pablo Correa Gómez <pabloyoyoista@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7649>

.ci/lib/common.py

@@ -76,6 +76,7 @@ def get_upstream_branch():
     return channels_cfg[channel]["branch_pmaports"]
 
 
+@cache
 def get_base_commit() -> str:
     """Get the base commit that can be compared with HEAD to get all commits in the
     given branch/merge request."""