Add lint for shellcheck, fix issues #158

	---
	name: CI

	'on':
	- push
	- pull_request

	env:
	GOPROXY: https://proxy.golang.org
	GOPATH: ${{ github.workspace }}/go

	permissions:
	contents: read

	jobs:
	build:
	runs-on: ubuntu-latest
	steps:
	- name: Set up go
	uses: actions/setup-go@v5
	with:
	go-version: '1.22.4'

	- uses: actions/checkout@v4
	with:
	path: git-sync

	- name: make all-build
	working-directory: git-sync
	run: \|
	make all-build

	test:
	runs-on: ubuntu-latest
	steps:
	- name: Set up go
	uses: actions/setup-go@v5
	with:
	go-version: '1.22.4'

	- uses: actions/checkout@v4
	with:
	path: git-sync

	- name: make test
	working-directory: git-sync
	run: \|
	make test

	lint:
	runs-on: ubuntu-latest
	steps:
	- name: Set up go
	uses: actions/setup-go@v5
	with:
	go-version: '1.22.4'

	- uses: actions/checkout@v4
	with:
	path: git-sync

	# We run the github action, even though this is duplicated in `make lint` below.
	# This is because the action gives easier-to-read output than the linter.
	# There is a risk of drift between the two, but this is only linting,
	# not runtime correctness!
	- name: golangci-lint
	uses: golangci/golangci-lint-action@v6
	with:
	working-directory: git-sync
	version: v1.59.0

	- name: make lint
	working-directory: git-sync
	run: \|
	make lint

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