A primary key error was occurring whenever a PatchSet was pushed
with a label overriding a sticky approval from a prior PatchSet.
The cause is that when changes are read from NoteDb, ApprovalCopier
doesn't have access to these new approvals.
This change adds an optional parameter to ApprovalCopier#copy,
dontCopy, which is used to block copying of approvals of the
given (account, label) pairs from any prior PatchSets.
Change-Id: I417deba5b42e38f498c879696998651597535659
9f07939a76