repo-updater: Reset Bitbucket Server external ids
requested to merge core/reset-bitbucket-server-external-ids into core/support-external-id-migrations
Created by: tsenart
This commit adds a migration that sets the external_id column of
Bitbucket Server repos to NULL so that the new Syncer can fill it in
with the new stable external IDs introduced in #3097.
This is required since we limited how the Syncer joins stored and sourced repos by name to only those stored repos that had the name completely unset: https://github.com/sourcegraph/sourcegraph/pull/3745/files#diff-358c3ac1d1a3c6cd1867f1808d3bdb88R192
This means that customers upgrading from 3.2 to 3.3 would have repos deleted and then re-created in a second syncer run.
Depends on #3848
Test plan: go test