Summary: Use both old dest file and partial file as basis data
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Currently, a file transfer can use only basis file: the old destination file,
the file in the partial dir, or an alternate basis from somewhere. This makes
it difficult to complete the delta-transfer of a large file if rsync keeps
getting interrupted and has to be restarted. If rsync transfers some literal
data (setting cleanup_got_literal) and is then interrupted, the partial file
will be kept, and the next run will not have the benefit of the later data in
the old destination file as basis data.
At a minimum, rsync should use both the old destination file and the partial
file as basis data. In some circumstances it might be beneficial to keep
multiple partial files rather than replacing the previous one, but I'm not sure
whether this is common enough to be worth implementing.