Limit the number of merge requests per project to avoid long seeds

This step was taking a long time because seed_fu creates N / 2 merge requests
for each repo, where N is the number of branches for that repo. At the time of
this writing, there are 234 branches on the gitlab-ce repo, leading to 117
merge requests.

db/fixtures/development/10_merge_requests.rb

 Gitlab::Seeder.quiet do
+  # Limit the number of merge requests per project to avoid long seeds
+  MAX_NUM_MERGE_REQUESTS = 10
+
   Project.all.reject(&:empty_repo?).each do |project|
-    branches = project.repository.branch_names
+    branches = project.repository.branch_names.sample(MAX_NUM_MERGE_REQUESTS * 2)
 
     branches.each do |branch_name|
       break if branches.size < 2