Use processed bytes for summary

[fd0]

What is the purpose of this change? What does it change?

When restic reads the backup from stdin, the number of bytes processed
was always displayed as zero. The reason is that the UI for the archive
uses the total bytes as returned by the scanner, which is zero for
stdin. So instead we keep track of the real number of bytes processed
and print that at the end.

Was the change discussed in an issue or in the forum before?

Closes #2136
Supersedes #2135

Checklist

• I have read the Contribution Guidelines
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo

[codecov-io]

Codecov Report

Merging #2138 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2138      +/-   ##
==========================================
- Coverage   50.69%   50.68%   -0.02%     
==========================================
  Files         176      176              
  Lines       14167    14169       +2     
==========================================
- Hits         7182     7181       -1     
- Misses       5938     5941       +3     
  Partials     1047     1047

Impacted Files	Coverage Δ
internal/ui/backup.go	0% <0%> (ø)	⬆️
internal/backend/rclone/backend.go	64.32% <0%> (-1.76%)	⬇️
internal/backend/b2/b2.go	79.54% <0%> (-1.14%)	⬇️
internal/archiver/blob_saver.go	97.53% <0%> (+4.93%)	⬆️

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[mholt]

I wonder if we could/should merge #1944 first and then include the JSON reporting in this change.

[fd0]

Hm, I think you're right.