ellipsis_truncate: pad a truncated string to the full width (1.4-maint)

[ThomasWaldmann]

Description

Backport of #10480.

ellipsis_truncate() pads a short string to the given width, but it did not pad a truncated one. swidth_slice() does not split a wide (e.g. CJK) character, so each of the two slices around the ... can be one cell short, and the truncated result could be up to 2 cells narrower than requested.

borg create --progress and borg extract --progress (which uses ellipsis_truncate() in ProgressIndicatorPercent on 1.4) repaint their line with a carriage return and rely on that padding to overwrite the previous line. With wide characters in the paths, the end of a longer previous line stayed visible, e.g. pwide/日...い名前_2.txtt.

The truncated string is now padded like the short one.

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.10%. Comparing base (084f3fb) to head (c13c9ee).
⚠️ Report is 1 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10481      +/-   ##
=============================================
- Coverage      82.11%   82.10%   -0.01%     
=============================================
  Files             38       38              
  Lines          11479    11480       +1     
  Branches        1808     1808              
=============================================
  Hits            9426     9426              
- Misses          1467     1468       +1     
  Partials         586      586              

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