Fix shallow fetch by checking out FETCH_HEAD

[asottile]

I don't think this was actually succeeding before:

$ bash -x t2.sh
+ set -euxo pipefail
+ git --version
git version 2.20.1 (Apple Git-117)
+ rm -rf repo1
+ git init repo1
Initialized empty Git repository in /private/tmp/x/repo1/.git/
+ cd repo1
+ git remote add origin https://github.com/asottile/pyupgrade
+ git -c protocol.version=2 fetch origin v1.20.1 --depth=1
remote: Enumerating objects: 38, done.
remote: Counting objects: 100% (38/38), done.
remote: Compressing objects: 100% (32/32), done.
remote: Total 38 (delta 4), reused 27 (delta 3), pack-reused 0
Unpacking objects: 100% (38/38), done.
From https://github.com/asottile/pyupgrade
 * tag               v1.20.1    -> FETCH_HEAD
+ git checkout v1.20.1
error: pathspec 'v1.20.1' did not match any file(s) known to git