ci: Fix identifying commits in MR

When a MR uses a private namespace, "origin" refers to that, and its
master branch may be outdated with regard to upstream master.

We are really only interested in checking the line changes from the commits
in the MR, so figure out the correct branch point instead.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/647

.gitlab-ci/run-eslint.sh

@@ -67,7 +67,10 @@ create_common() {
 }
 
 if [ "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
-  commit_range=origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME...$CI_COMMIT_SHA
+  git fetch $CI_MERGE_REQUEST_PROJECT_URL.git $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+  branch_point=$(git merge-base HEAD FETCH_HEAD)
+  commit_range=$branch_point...$CI_COMMIT_SHA
+
   list_commit_range_additions $commit_range > $LINE_CHANGES
 
   # Don't bother with running lint when no JS changed