Fix a recurring issue after the migration of external ids from ReviewDb
to NoteDb failing with the error 'Email <email> is already assigned to
account <>' and thus failing LDAP authentication.
The LDAP authentication may or may not have generated the preferred
email record in the main 'gerrit:<>' account. Consequently, users may
have added their external id as additional 'mailto:<>' record. That
was a normally supported use-case and, previously in v2.15, did not
create any problems or conflicts.
Starting with v2.16 the extra check for unique e-mails did go a bit too
far and blocked the ability to have both the 'mailto:' record and having
the same value also as preferred e-mail, which is incorrect because it is
a perfectly valid use-case (see Change-Id: Ie4c677365cd).
When linking or updating the preferred e-mail to an existing account,
accept duplications only if both external ids belongs to the target
account id. Because both e-mails (preferred e-mail and 'mailto:' record)
belongs to the same account, they are not really duplicated but just a
legacy of the way Gerrit was used to indicate an external 'mailto:'
identity that has been selected as primary e-mail afterwards.
Add unit-test that consistently reproduce the issue that many people
have reported after having migrated the accounts and external-ids to
All-Users.
NOTE: Even though the issue has been reported on the LDAP authentication
use-case, the problem lies in AccountManager and is more generic.
Fix the consistency checker for the e-mail external ids when
multiple duplicate entries are associated with the same account.
Bug: Issue 11246
Bug: Issue 9001
Change-Id: I78bc82faa2761bc0e56a9fa54a94225c82317275
91180b417e