Creating a new Merge Request times out if there are too many commits

Even after the improvements of !2705 (merged) it is still too easy to make GitLab time out while creating a Merge Request. Steps to reproduce:

  • import the Linux kernel
  • create a branch b1 pointing to v2.6.13-rc4
  • create a branch b2 pointing to v2.6.14
  • create a MR to merge b2 into b1

cc @yorickpeterse