Link mr commits/diffs to target project

Now when changes in merge requests stored in target project we can
always link commits and diffs to target project. So even people without
access to fork can browse commits and diffs

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

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

-= render "projects/commits/commits", project: @merge_request.source_project
+= render "projects/commits/commits", project: @merge_request.project

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

 - if @merge_request_diff.collected?
-  = render "projects/diffs/diffs", diffs: @merge_request.diffs, project: @merge_request.source_project
+  = render "projects/diffs/diffs", diffs: @merge_request.diffs, project: @merge_request.project
 - elsif @merge_request_diff.empty?
   .nothing-here-block Nothing to merge from #{@merge_request.source_branch} into #{@merge_request.target_branch}
 - else