Add scripts to simplify offline builds of docs and CMake

tools/offline/scripts/use_git_cache.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+# needs to be sourced
+if [ -z "${GITHUB_PROXY_DIR}" ]
+then
+    echo "Must set GITHUB_PROXY_DIR environment variable"
+    exit 1
+fi
+
+export GIT_CONFIG_COUNT=1
+export GIT_CONFIG_KEY_0=url.$GITHUB_PROXY_DIR/.insteadOf
+export GIT_CONFIG_VALUE_0=git://github.com/
+
+echo "Redirecting git://github.com urls to local cache..."
+
+function deactivate_git_cache {
+    echo "Removing git://github.com redirect..."
+    unset GIT_CONFIG_COUNT
+    unset GIT_CONFIG_KEY_0
+    unset GIT_CONFIG_VALUE_0
+}