Do not prune objects during 'git gc'

This is a workaround to reduce the impact of
https://gitlab.com/gitlab-org/gitlab-ce/issues/13524 .

lib/gitlab_projects.rb

@@ -288,7 +288,7 @@ class GitlabProjects
       $logger.error "gc failed: destination path <#{full_path}> does not exist."
       return false
     end
-    cmd = %W(git --git-dir=#{full_path} gc)
+    cmd = %W(git --git-dir=#{full_path} gc --no-prune)
     system(*cmd)
   end
 end