Fetch refs/pull-requests/<number>/from intermittently takes several minutes to become available

When a pull request is created in Stash, there is a ref in the format of refs/pull-requests/<number>/from which becomes available while the PR is open.

The process of how to do this was from these sources:

• http://blogs.atlassian.com/2014/08/how-to-fetch-pull-requests/
• http://mark.shropshires.net/blog/checking-out-pull-requests-stash-and-github

The method of how we do this is from the Atlassian blog post above:

# Example for a PR with number "239"
git fetch origin pull-requests/239/from:pr/239/from
git checkout pr/239/from

However, recently we have noticed that this ref occasionally takes upto 5 minutes to become available after the PR is created. When this happens our automated CI returns with an error like:
fatal: Couldn't find remote ref pull-requests/<number>/from (where <number> is the PR number)

A similar error occurs if we try to fetch the specific hash we know is in the pull request:
fatal: reference is not a tree: XXXXXX (where XXXXX is the specific hash)

If we wait up to 5 minutes when this happens, the refs of both formats above become available. We are hosting stash on a pretty big machine.

This issue is making CI difficult, as we block pull requests until the CI job is run and successful. Waiting around for 5 or more minutes before we can start any jobs gets in the way of engineers.