Merge branch 'fix_500-commit-null' into 'master'

Don't try to mark broken MR as mergable

In case merge request is broken, we shouldn't check if the sha
is mergable, as it will be null, and there's no point, as we know
that it's not mergable.

See merge request !2783

app/models/merge_request.rb

@@ -241,7 +241,7 @@ class MergeRequest < ActiveRecord::Base
     return unless unchecked?
 
     can_be_merged =
-      project.repository.can_be_merged?(source_sha, target_branch)
+      !broken? && project.repository.can_be_merged?(source_sha, target_branch)
 
     if can_be_merged
       mark_as_mergeable