Add LATEST_TAG variable

doc/update/patch_versions.md

 # Universal update guide for patch versions
 *Make sure you view this [upgrade guide](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/update/patch_versions.md) from the `master` branch for the most up to date instructions.*
 
-For example from 6.2.0 to 6.2.1, also see the [semantic versioning specification](http://semver.org/).
+For example from 7.14.0 to 7.14.3, also see the [semantic versioning specification](http://semver.org/).
 
 ### 0. Backup
 
@@ -23,17 +23,16 @@ sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
 cd /home/git/gitlab
 sudo -u git -H git fetch --all
 sudo -u git -H git checkout -- Gemfile.lock db/schema.rb
-sudo -u git -H git checkout LATEST_TAG
+LATEST_TAG=$(git describe --tags `git rev-list --tags --max-count=1`)
+sudo -u git -H git checkout $LATEST_TAG -b $LATEST_TAG
 ```
 
-Replace LATEST_TAG with the latest GitLab tag you want to upgrade to, for example `v6.6.3`.
-
 ### 3. Update gitlab-shell to the corresponding version
 
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch
-sudo -u git -H git checkout v`cat /home/git/gitlab/GITLAB_SHELL_VERSION`
+sudo -u git -H git checkout v`cat /home/git/gitlab/GITLAB_SHELL_VERSION` -b v`cat /home/git/gitlab/GITLAB_SHELL_VERSION`
 ```
 
 ### 4. Install libs, migrations, etc.