rebase-pkg: Support native packages

Native packages don't have an upstream branch, so check its existence
before trying to branch from them.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

rebase-scripts/rebase-pkg

@@ -83,8 +83,11 @@ else
     PRJ_ID=${PRJ_ID_ORI}
 fi
 
-# 3. Branch upstream/trixie from upstream/bookworm
-python-gitlab project-branch create --project-id ${PRJ_ID} --branch ${NEW_UPSTREAM_BRANCH} --ref ${OLD_UPSTREAM_BRANCH}
+# 3. Branch upstream/trixie from upstream/bookworm if it is present
+if python-gitlab project-branch get --project-id ${PRJ_ID} --name ${OLD_UPSTREAM_BRANCH}
+then
+    python-gitlab project-branch create --project-id ${PRJ_ID} --branch ${NEW_UPSTREAM_BRANCH} --ref ${OLD_UPSTREAM_BRANCH}
+fi
 
 # 4. Branch debian/trixie from debian/bookworm
 python-gitlab project-branch create --project-id ${PRJ_ID} --branch ${NEW_DEBIAN_BRANCH} --ref ${OLD_DEBIAN_BRANCH}