(not for master) CI: hardcode get_upstream_branch (MR 5070)

Set it to master_staging_systemd, and bypass the usual logic of looking
into pmaports.cfg and channels.cfg (of master branch!). This avoids
adding it to channels.cfg of the master branch.

.ci/lib/common.py

@@ -56,34 +56,8 @@ def get_upstream_branch():
 
         :returns: branch name, e.g. "v20.05" """
 
-    # Prefer gitlab CI target branch name if it's set (i.e. running in gitlab CI)
-    if target_branch := os.environ.get("CI_MERGE_REQUEST_TARGET_BRANCH_NAME"):
-        return target_branch
-
-    global cache
-    if "upstream_branch" in cache:
-        return cache["upstream_branch"]
-
-    # Get channel (e.g. "stable") from pmaports.cfg
-    # https://postmarketos.org/pmaports.cfg
-    pmaports_dir = get_pmaports_dir()
-    pmaports_cfg = configparser.ConfigParser()
-    pmaports_cfg.read(f"{pmaports_dir}/pmaports.cfg")
-    channel = pmaports_cfg["pmaports"]["channel"]
-
-    # Get branch_pmaports (e.g. "v20.05") from channels.cfg
-    # https://postmarketos.org/channels.cfg
-    channels_cfg_str = run_git(["show", "upstream/master:channels.cfg"])
-    channels_cfg = configparser.ConfigParser()
-    channels_cfg.read_string(channels_cfg_str)
-    assert channel in channels_cfg, \
-        f"Channel '{channel}' from pmaports.cfg in your branch is unknown." \
-        " This appears to be an old branch, consider recreating your change" \
-        " on top of master."
-
-    ret = channels_cfg[channel]["branch_pmaports"]
-    cache["upstream_branch"] = ret
-    return ret
+    # HACK for master_staging_systemd branch
+    return "master_staging_systemd"
 
 
 def get_changed_files(removed=True):