checkpatch,gitlab-ci: let checkpatch script compare against latest upstream master

When opening a merge request from a fork of NetworkManager, then the
pipeline runs with the a checkout of the fork. That means, checkpatch
would compare the branch against "master" (or "nm-x-y" stable branches)
of the fork, instead of upstream.

That doesn't seem too useful. Instead, also add upstream NetworkManager
as git remote, fetch the branches, and use the branches from there as
base for checkpatch.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/255
(cherry picked from commit 3019648b4b)
(cherry picked from commit 9a37702311)

.gitlab-ci.yml

@@ -73,7 +73,7 @@ checkpatch:
   stage: test
   script:
     - date '+%Y%m%d-%H%M%S'; dnf install -y git
-    - date '+%Y%m%d-%H%M%S'; contrib/scripts/checkpatch-feature-branch.sh 2>&1 | tee checkpatch-out.txt
+    - date '+%Y%m%d-%H%M%S'; NM_CHECKPATCH_FETCH_UPSTREAM=1 contrib/scripts/checkpatch-feature-branch.sh 2>&1 | tee checkpatch-out.txt
   allow_failure: true
   artifacts:
     when: on_failure

contrib/scripts/checkpatch-feature-branch.sh

@@ -15,6 +15,12 @@ if printf '%s' "$HEAD" | grep -q '\.\.'; then
 else
     BASE_REF="refs/remotes/origin/"
 
+    if [ "$NM_CHECKPATCH_FETCH_UPSTREAM" == 1 ]; then
+        git remote add nm-upstream https://gitlab.freedesktop.org/NetworkManager/NetworkManager.git
+        git fetch nm-upstream || die "failure to fetch from https://gitlab.freedesktop.org/NetworkManager/NetworkManager.git"
+        BASE_REF="refs/remotes/nm-upstream/"
+    fi
+
     # the argument is only a single ref (or the default "HEAD").
     # Find all commits that branch off one of the stable branches or master
     # and lead to $HEAD. These are the commits of the feature branch.