MergeabilityChecker: Fix isStale() optimization

Change copy constructor is missing to copy the value of lastSha1MergeTested.
MergeabilityChecker relies on this value to be set to skip unnecessary
mergeability status reevaluation and no-op database and index operations.

Change-Id: Id4ef8388b026a68d5f1d97ee89c526a04797c5dc