Perfectly normal repository causes gitaly to hang

Summary

While generating the pages list for a moderately-sized Wiki, gitaly starts gobbling up gigabytes of memory and is eventually killed. Likely relatedly, pushing the repository to gitlab.com hung.

Steps to reproduce

I have found two manifestations of the issue:

• Create a new project. Push this repository to the Wiki and try viewing the Wiki pages list. The request will time out; looking at the server logs you will likely see that the gitaly process was killed by the OOM killer.

• Try pushing the same repository to gitlab.com. This will fail with,

  $ git push git@gitlab.com:bgamari/ghc-wiki master
  Enumerating objects: 81883, done.
  Counting objects: 100% (81883/81883), done.
  Delta compression using up to 4 threads.
  Compressing objects: 100% (21457/21457), done.
  Writing objects: 100% (81883/81883), 18.21 MiB | 3.22 MiB/s, done.
  Total 81883 (delta 60224), reused 81715 (delta 60134)
  remote: Resolving deltas: 100% (60224/60224), done.
  remote: GitLab: Push operation timed out
  remote: 
  remote: Timing information for debugging purposes:
  remote: Running checks for ref: master
  remote: Checking if you are allowed to push... (13.8ms)
  remote: Checking if default branch is being deleted... (0.38ms)
  remote: Checking if you are force pushing to a protected branch... (2.05ms)
  remote: Checking if you are allowed to push to the protected branch... (6.32ms)
  remote: Scanning repository for blobs stored in LFS and verifying their files have been uploaded to GitLab... (23.68ms)
  remote: Checking if branch follows the naming patterns defined by the project... (8.0ms)
  remote: Validating diff contents... (cancelled after 46800.76ms)
  To gitlab.com:bgamari/ghc-wiki
   ! [remote rejected]     master -> master (pre-receive hook declined)
  error: failed to push some refs to 'git@gitlab.com:bgamari/ghc-wiki'

Example Project

https://github.com/bgamari/ghc-wiki-import

What is the current bug behavior?

Gitaly blows up.

What is the expected correct behavior?

Gitaly shouldn't blow up.

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Output of checks

This happens on gitlab.com and our local 11.7 installation.

Possible fixes

No idea.