CI: Fetch minimal history before GitHub push

The trigger_snap_builds job fails when pushing to GitHub because GitLab
uses shallow clones by default. GitHub may expect objects that aren't
present in the shallow clone.

Fix this by fetching only the commits needed: first get GitHub's current
master (depth=1), then fetch from origin only commits since that date.
This is much faster than fetching the entire history.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

.gitlab-ci.yml

@@ -978,5 +978,7 @@ trigger_snap_builds:
       git config --global --add safe.directory $CI_PROJECT_DIR
       cd $CI_PROJECT_DIR
       git remote add github git@github.com:sjg20/u-boot-concept.git || true
+      git fetch github master --depth=1
+      git fetch origin --shallow-since=$(git log -1 --format=%ci github/master)
       git push github HEAD:master --force
       echo "GitHub push completed"