Remove 8.0-only steps from the 8.1 update guide

Also adds a preamble note that a working 8.0 installation (i.e.,
gitlab-git-http-server, updated Nginx/Apache configs) is required before
proceeding.

[ci skip]

doc/update/8.0-to-8.1.md

 # From 8.0 to 8.1
 
+**NOTE:** GitLab 8.0 introduced several significant changes related to
+installation and configuration which *are not duplicated here*. Be sure you're
+already running a working version of 8.0 before proceeding with this guide.
+
 ### 0. Double-check your Git version
 
 **This notice applies only to /usr/local/bin/git**
@@ -66,58 +70,7 @@ sudo -u git -H git fetch
 sudo -u git -H git checkout v2.6.5
 ```
 
-### 5. Install gitlab-git-http-server
-
-First we download Go 1.5 and install it into `/usr/local/go`:
-
-```bash
-curl -O --progress https://storage.googleapis.com/golang/go1.5.linux-amd64.tar.gz
-echo '5817fa4b2252afdb02e11e8b9dc1d9173ef3bd5a  go1.5.linux-amd64.tar.gz' | shasum -c - && \
-  sudo tar -C /usr/local -xzf go1.5.linux-amd64.tar.gz
-sudo ln -sf /usr/local/go/bin/{go,godoc,gofmt} /usr/local/bin/
-rm go1.5.linux-amd64.tar.gz
-```
-
-Now we download `gitlab-git-http-server` and install it in `/home/git/gitlab-git-http-server`:
-
-```bash
-cd /home/git
-sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-git-http-server.git
-cd gitlab-git-http-server
-sudo -u git -H make
-```
-
-Make sure your unicorn.rb file contains a 'listen' line for
-'127.0.0.1:8080' and that this line is not commented out.
-
-```
-cd /home/git/gitlab
-grep ^listen config/unicorn.rb
-
-# If there is no 'listen' line for 127.0.0.1:8080, add it:
-sudo -u git tee -a config/unicorn.rb <<EOF
-listen "127.0.0.1:8080", :tcp_nopush => true
-EOF
-```
-
-If your Git repositories are in a directory other than `/home/git/repositories`,
-you need to tell `gitlab-git-http-server` about it via `/etc/default/gitlab`.
-See `lib/support/init.d/gitlab.default.example` for the options.
-
-### 6. Copy secrets
-
-The `secrets.yml` file is used to store keys to encrypt sessions and encrypt secure variables.
-When you run migrations make sure to store it someplace safe.
-Don't store it in the same place as your database backups,
-otherwise your secrets are exposed if one of your backups is compromised.
-
-```
-cd /home/git/gitlab
-sudo -u git -H cp config/secrets.yml.example config/secrets.yml
-sudo -u git -H chmod 0600 config/secrets.yml
-```
-
-### 7. Install libs, migrations, etc.
+### 5. Install libs, migrations, etc.
 
 ```bash
 cd /home/git/gitlab
@@ -138,7 +91,7 @@ sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS
 sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
 ```
 
-### 8. Update config files
+### 6. Update configuration files
 
 #### New configuration options for `gitlab.yml`
 
@@ -148,41 +101,12 @@ There are new configuration options available for [`gitlab.yml`](config/gitlab.y
 git diff origin/8-0-stable:config/gitlab.yml.example origin/8-1-stable:config/gitlab.yml.example
 ```
 
-#### New Nginx configuration
-
-Because of the new `gitlab-git-http-server` you need to update your Nginx
-configuration.  If you skip this step 'git clone' and 'git push' over HTTP(S)
-will stop working.
-
-View changes between the previous recommended Nginx configuration and the
-current one:
-
-```sh
-# For HTTPS configurations
-git diff origin/8-0-stable:lib/support/nginx/gitlab-ssl origin/8-1-stable:lib/support/nginx/gitlab-ssl
-
-# For HTTP configurations
-git diff origin/8-0-stable:lib/support/nginx/gitlab origin/8-1-stable:lib/support/nginx/gitlab
-```
-
-If you are using Apache instead of NGINX please see the updated [Apache templates](https://gitlab.com/gitlab-org/gitlab-recipes/tree/master/web-server/apache).
-Also note that because Apache does not support upstreams behind Unix sockets you will need to let gitlab-git-http-server listen on a TCP port. You can do this via [/etc/default/gitlab](https://gitlab.com/gitlab-org/gitlab-ce/blob/8-0-stable/lib/support/init.d/gitlab.default.example#L34).
-
-### 9. Use Redis v2.4.0+
-
-Previous versions of GitLab allowed Redis versions >= 2.0 to be used, but
-Sidekiq jobs could fail due to lack of support for the SREM command. GitLab
-8.0 now checks that Redis >= 2.4.0 is used. You can check your Redis version
-with the following command:
-
-    redis-cli info | grep redis_version
-
-### 10. Start application
+### 7. Start application
 
     sudo service gitlab start
     sudo service nginx restart
 
-### 11. Check application status
+### 8. Check application status
 
 Check if GitLab and its environment are configured correctly:
 
@@ -214,8 +138,4 @@ If you have more than one backup `*.tar` file(s) please add `BACKUP=timestamp_of
 
 ### "You appear to have cloned an empty repository."
 
-If you see this message when attempting to clone a repository hosted by GitLab,
-this is likely due to an outdated Nginx or Apache configuration, or a missing or
-misconfigured `gitlab-git-http-server` instance. Double-check that you correctly
-completed [Step 5](#5-install-gitlab-git-http-server) to install the daemon and
-[Step 8](#new-nginx-configuration) to reconfigure Nginx.
+See the [7.14 to 8.0 update guide](7.14-to-8.0.md#troubleshooting).