If Gerrit is aliased under multiple DNS names and/or HTTP ports we may
have asked Google Accounts to generate us different OpenID strings for
the same user identity. By looking for an existing Google Account key
using the email address we can avoid creating a duplicate account for
the user if they login to the same database, but through a CNAME alias.
Signed-off-by: Shawn O. Pearce <sop@google.com>
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