Inform user when comparing 2 version with different base

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

app/assets/stylesheets/pages/merge_requests.scss

@@ -401,8 +401,13 @@
     padding: 16px;
   }
 
+  .content-block {
+    padding: $gl-padding-top $gl-padding;
+  }
+
   .comments-disabled-notif {
-    padding: 10px 16px;
+    border-top: 1px solid $border-color;
+
     .btn {
       margin-left: 5px;
     }
@@ -413,10 +418,6 @@
     margin: 0 7px;
   }
 
-  .comments-disabled-notif {
-    border-top: 1px solid $border-color;
-  }
-
   .dropdown-title {
     color: $gl-text-color;
   }

app/views/projects/merge_requests/show/_versions.html.haml

@@ -64,6 +64,13 @@
                       #{@merge_request.target_branch} (base)
                     .monospace #{short_sha(@merge_request_diff.base_commit_sha)}
 
+    - if @start_version && @start_version.base_commit_sha != @merge_request_diff.base_commit_sha
+      .content-block
+        Selected versions have different base commits.
+        Changes will include
+        = link_to namespace_project_compare_path(@project.namespace, @project, from: @start_version.base_commit_sha, to: @merge_request_diff.base_commit_sha) do
+          new commits
+        from #{@merge_request.target_branch}
     - unless @merge_request_diff.latest? && !@start_sha
       .comments-disabled-notif.content-block
         = icon('info-circle')