Add the git-mirror script uses on Jenkins

This is run on one of Jenkins Linux machines to mirror all the
SF repos for faster cloning and builds when SF is offline.

update_git_mirror.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+mirror_dirs='fgdata fgmeta flightgear simgear getstart windows-3rd-party fgrun'
+
+pushd /var/lib/git
+
+for dir in $mirror_dirs
+do
+	repo_name=$dir.git
+	if [ ! -d "$repo_name" ]; then
+		echo "Doing initial clone"
+		git clone --mirror git://git.code.sf.net/p/flightgear/$dir
+	fi
+
+	pushd $repo_name
+	echo "Updating in $PWD"
+	git remote update
+	popd
+done
+
+popd
+