Use a single progress bar for the internal block copy operation (#762367)

As part of the internal block copy operation 5 initial ranges of blocks
are copied using different block sizes to determine the fastest.  Then
the remainder is copied using the fastest block size.  Each of these
copies reports progress independently, so during the benchmarking phase
the progress bar flashes 5 times as it goes from 0 to 100% in a fraction
of a second, before showing the progress of the remainder.

This looks bad, so report a single progress bar for all the ranges of
blocks copied in a single copy operation.

Already have variables done and length which track progress within each
copied range; and total_done which records amount copied in previous
ranges.  Just add total_length to allow overall progress to be reported.

Bug 762367 - Use a single progress bar for the whole of the internal
             copy operation