Closing a branch upon merging a Pull Request is not cleaning up the reflog

When you select 'close branch after the pull request is merged' and the merge the pull request the reflog is not being cleaned up. This means that any future branches with that name as path prefix cannot be successfully pushed to due to a git error.

Steps to reproduce:

1. Create a branch from master called "foo"
2. Create a commit on foo and push it to Bitbucket
3. Create a PR from foo to master, selecting close branch after merge
4. Merge the PR
5. Create a branch locally called "foo/my-new-branch-name"
6. Attempt to push a commit to "foo/my-new-branch-name" on bitbucket

You will see an error from git that looks something like
error: unable to create directory for ./logs/refs/heads/foo/my-new-branch-name