Import of an archive in instant migration with inconsistent user display name fails.

Issue Summary

User importer uses full export id ('username|display_name||NORMAL') as an key to match users that it has already imported while the database uses only username to do the same matching. That leads to a failure when there are two records for the same user with same username but different display names.

Steps to Reproduce

1. Have some unknown issue in Crowd (or may be it's in Bitbucket itself?) configuration but it works perfectly fine otherwise
2. Have some pull requests where a user X is participant
3. Have different display name for user X in Crowd tables and in sta_user_normal
4. Perform export of the repository that contains the aforementioned pull requests
5. Try to import repository on another instance.

Expected Results

Import completes successfully. 

Actual Results

Exception executing callback 'importEntry' on 'class com.atlassian.stash.internal.migration.entity.pull.PullRequestImporter': 'Could not import pull request 'PROJECT/repository[100500]#666': persisting to the database failed: 'user id' already mapped to 'username|display_name||NORMAL'

Workaround

Not available yet.