MergeRequests services

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Fix mr update issue if merge_request_diff missing

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'improve/mr_service' into 'master'

Improve merge request accepting logic

Add locked state to merge request during `accept` action.
Prevents mr being `triggered as merged` twice (hooks, emails etc)
Fixes #998 #729