02eea55 added code to load a buffer of data from a file being backup up to detect files that have been truncated to zero after manifest generation. This mechanism can also be used to detect files that have not changed since the prior backup.
If the result of the file copy fits into a single buffer, then the size and checksum can be compared to the prior file before anything gets stored. If the file matches then it is referenced to the file in to prior backup.
The size that can be compared for normal copies is limited by the buffer size but for block incremental it works with any size file since there is no output from block incremental when the file is identical.
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