Cross-repository pull request merges sometimes fail to fetch objects after upgrading to Stash 3.9

After upgrading to Stash 3.9, some pull requests hang for long periods trying to display the diff. When the diff is eventually displayed, the pull request indicates the diff has conflicts but none are shown.

This can also manifest as a lengthy (~5 minute) delay merging pull requests, after which the pull request ultimately fails to merge. A "Server unreachable" error will be shown after ~1 minute due to STASH-7360, but refreshing shows that the merge didn't ever complete.

This regression is only possible on cross-repository pull requests. Pull requests between branches within a single repository use a different process which does not have this issue.