chore: save HEAD when git-import-patches runs (#17824)

script/git-import-patches

@@ -17,6 +17,12 @@ def main(argv):
       help="use 3-way merge to resolve conflicts")
   args = parser.parse_args(argv)
 
+  # save the upstream HEAD so we can refer to it when we later export patches
+  git.update_ref(
+      repo='.',
+      ref='refs/patches/upstream-head',
+      newvalue='HEAD'
+  )
   git.am(
       repo='.',
       patch_data=patch_from_dir(args.patch_dir),