add mirror repo example

examples/git.example.com/org/mirror-project/deploy.sh

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+#!/bin/bash
+set -u
+set -e
+
+my_mirror=git.example.com/org/project
+echo "Mirroring ${GIT_REPO_ID}#${GIT_REF_NAME} to $my_mirror ..."
+
+# See the Git Credentials Cheat Sheet
+# https://coolaj86.com/articles/vanilla-devops-git-credentials-cheatsheet/
+if [[ ! -d "$HOME/git-mirrors/${GIT_REPO_ID}" ]]; then
+    mkdir -p "$(dirname "$HOME/git-mirrors/${GIT_REPO_ID}")"
+    git clone "${GIT_CLONE_URL}" "$HOME/git-mirrors/${GIT_REPO_ID}"
+    pushd "$HOME/git-mirrors/${GIT_REPO_ID}"
+      git remote add mirror ssh://git@${my_mirror}.git
+      git fetch --all
+    popd
+fi
+
+pushd "${HOME}/git-mirrors/${GIT_REPO_ID}"
+    # get a random branch name
+    my_tmp="$(xxd -l8 -ps /dev/urandom)"
+    # get clean
+    git reset --hard HEAD
+    # make a temp branch
+    git checkout -b "${my_tmp}"
+    # delete the existing branch, if any
+    git branch -D "${GIT_REF_NAME}"
+    # get updated
+    git fetch --force origin
+    # get the latest branch
+    git checkout --force "origin/${GIT_REF_NAME}" -b "${GIT_REF_NAME}"
+    # delete the temp branch
+    git branch -D "${my_tmp}"
+    # get the latest of this branch
+    git pull --force
+    # overwrite the mirror's copy
+    git push --force mirror "${GIT_REF_NAME}"
+popd