/changes/id/detail can be slow partly because the Mute and Unmute
action descriptions need to check if the current user has set the
relevant labels on the change.
The lookups used to be scanning all stars applied to a change by all
users, filtering those files for the relevant label, and then testing
if the user is contained in that set. This is fairly backwards.
The caller knows exactly which user it needs to read. Read only that
user's ref from All-Users, parse only that user's label blob, and test
if the blob contains the relevant label of interest.
Change-Id: I0e8abac313e0067f6fa4654ca77b59a86cebed16
93477baf6a