Merging PR from fork to main repo gives 500 error

Issue Summary

Unable to access PR after merge from fork to main.

Steps to Reproduce

Replication on Bitbucket 7.7.1 instance:

1. Created a repository called new-repo, here: http://localhost:6771/b771/projects/FIR/repos/new-repo/browse. It has all the branches.
2. Then forked it in my username ajaitly, called new-repo-fork: http://localhost:6771/b771/users/ajaitly/repos/new-repo-fork/browse
3. Created a PR between branch bug/ARGUS-23284 from the fork repo, to the master branch in the main repo:
   
4. Merged the PR, the option for Delete source after merging is checked
   
5. Tried to access the Diff tab of the PR, got the same error:
   

• The Overview and Builds tab are fine, even the Commits tab. Error occurs only when we open that commit, or any other commit in that PR and while accessing the Diff tab.
• This issue occurs with other branches as well like feature type branch for example.

Expected Results

We should be able to see all the tabs of the PR successfully.

Actual Results

• We get an "Couldn't find ref error. The remote hung up unexpectedly" on the UI.
• The below exception is thrown in the atlassian-bitbucket.log file:

  2021-06-09 10:08:00,236 DEBUG [http-nio-6771-exec-5] ajaitly @3AERBSx608x979x0 1bdz5uq 0:0:0:0:0:0:0:1 "GET /rest/api/latest/projects/FIR/repos/avengers/pull-requests/3/changes HTTP/1.1" c.a.s.i.r.e.ServiceExceptionMapper Mapping ServiceException to REST response 500
  com.atlassian.bitbucket.scm.CommandFailedException: '/usr/bin/git fetch /Users/ajaitly/bitbucket-home/atlassian-bitbucket-7.7.1/shared/data/repositories/4 refs/heads/bug/ARGUS-23284:' exited with code 128 saying: fatal: couldn't find remote ref refs/heads/bug/ARGUS-23284
  fatal: the remote end hung up unexpectedly
  

  After a while when you try to access, it works fine. Attaching the full log file:

Workaround

If we try to access it after a while it may work. This issue is fixed in 7.12.0 and onwards, you can upgrade to that version or above that