.github: Allow specifying a start number for generated patches

When an action generates a couple of patches separately, then it might
be a good idea to specify a numbering, so applying the patches is done
in the desired order. Without that, all the generated patches would
start with "0001-" prefix.

sdk_container/src/third_party/coreos-overlay/.github/workflows/common.sh

@@ -129,7 +129,7 @@ function generate_patches() {
   git commit -a -m "${CATEGORY_NAME}: Upgrade ${PKGNAME_DESC} ${VERSION_OLD} to ${VERSION_NEW}"
 
   # Create a patch for the main ebuilds.
-  git format-patch -1 HEAD
+  git format-patch --start-number "${START_NUMBER:-1}" -1 HEAD
   popd || exit
 }