Error deploying to Heroku, failed to run repack
This is likely an issue caused by shallow clones. When you don't have a full history, the tree cannot be fully traversed resulting in dangling commits. This often occurs with CI systems where CI does a shallow clone to save on bandwidth and/or latency.
The best course of action to take is to avoid shallow clones.
If a full clone and force push doesn't do the trick, you may need to reset your repo. Resetting your repo re-initializes your app's repo to a bare repo. Your running application will not be affected. There is a utility plugin for resetting your repo on Heroku here:
https://github.com/heroku/heroku-repo
Once installed, run heroku repo:reset and then push again.
If the above techniques don't work, please log a support ticket.