Create ref for every merge request. Add documentation for fetch merge requests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request_diff.rb

@@ -161,8 +161,8 @@ class MergeRequestDiff < ActiveRecord::Base
   def compare_result
     @compare_result ||=
       begin
-        # Update ref if merge request is from fork
-        merge_request.fetch_ref if merge_request.for_fork?
+        # Update ref for merge request
+        merge_request.fetch_ref
         # Get latest sha of branch from source project
         source_sha = merge_request.source_project.commit(source_branch).sha

doc/workflow/merge_requests.md

+# Merge Requests
+
+Merge requests allow you to exchange changes you made to source code
+
+## Checkout merge requests locally
+
+
+Locate the section for your GitLab remote in the `.git/config` file. It looks like this:
+
+```
+[remote "origin"]
+  url = https://gitlab.com/gitlab-org/gitlab-ce.git
+  fetch = +refs/heads/*:refs/remotes/origin/*
+```
+
+Now add the line `fetch = +refs/merge-requests/*/head:refs/remotes/origin/merge-requests/*` to this section. 
+
+It should looks like this: 
+
+```
+[remote "origin"]
+  url = https://gitlab.com/gitlab-org/gitlab-ce.git
+  fetch = +refs/heads/*:refs/remotes/origin/*
+  fetch = +refs/merge-requests/*/head:refs/remotes/origin/merge-requests/*
+```
+
+Now you can fetch all the merge requests requests:
+
+```
+$ git fetch origin
+From https://gitlab.com/gitlab-org/gitlab-ce.git
+ * [new ref]         refs/merge-requests/1/head -> origin/merge-requests/1
+ * [new ref]         refs/merge-requests/2/head -> origin/merge-requests/2
+...
+```
+
+To check out a particular merge request:
+
+```
+$ git checkout origin/merge-requests/1
+```