Viewing old pull requests sometimes shows "Activity not found"

When viewing old pull requests, sometimes a recurring "Activity not found" error is displayed when loading the overview. This is caused by commits which have comments on them being garbage collected, due to a configuration bug in Stash 1.3. That issue was fixed in 2.1, but some pull requests had already had their commits removed.

The removed commits will prevent Stash from displaying diff context for comment activities, but it should not prevent viewing the pull request itself.