Add update-subtrees.sh based on script from higan

update-subtrees.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# Because git subtree doesn't provide an easy way to automatically merge changes
+# from upstream, this shell script will do the job instead.
+# If you don't have a POSIX-compatible shell on your system, feel free to use
+# this as a reference for what commands to run, rather than running it directly.
+
+# Change to the directory containing this script, or exit with failure
+# to prevent git subtree scrawling over some other repo.
+cd "$(dirname "$0")" || exit 1
+
+# Merge changes from the libco repository.
+git subtree pull --prefix=libco https://github.com/higan-emu/libco.git master