diff: add --stats, showing a summary of the differences

[ThomasWaldmann]

Fixes #796.

borg diff so far only reported per-item changes. This adds -s / --stats,
printing an aggregated summary after the per-path output:

Added items: 23
Removed items: 2
Changed items: 315
Added chunk volume: 53.70 MB
Removed chunk volume: 51.10 MB

• Added / removed items only exist in ARCHIVE2 / ARCHIVE1, changed items
  exist in both archives but differ.
• Added / removed chunk volume sum up the size of the content chunks
  added/removed by all of these items (a modified file contributes both its new
  and its old chunk bytes).
• --content-only narrows the counts the same way it narrows the output.

If the archives were created with different chunker params, borg compares the
content byte by byte and cannot tell by how much it changed. Such items
contribute no byte counts, so the totals are not silently misleading — an
additional line reports how many there are:

Items with unknown size changes: 1

The text summary goes to the borg.output.stats logger (stderr), like the
--stats output of other commands, so stdout stays clean for piping. With
--json-lines, the summary is emitted as a final line of its own shape
instead, so existing per-path parsers are unaffected:

{"stats": {"added_chunk_volume": 242000, "added_items": 2, "changed_items": 1, "removed_chunk_volume": 105000, "removed_items": 2, "unknown_size_items": 0}}

The change filtering that the text and JSON output paths each did separately is
now done once in reported_changes(), feeding both the output and the stats, so
the summary counts exactly what was printed.

Tests cover the text summary, metadata-only changes counting as changed items,
the JSON Lines summary, the unknown-size path, and that nothing is printed
without the flag. The diff epilog documents all of the above.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.72%. Comparing base (e861c72) to head (f554346).
⚠️ Report is 11 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10327      +/-   ##
==========================================
+ Coverage   87.65%   87.72%   +0.07%     
==========================================
  Files         103      103              
  Lines       18716    18756      +40     
  Branches     2881     2889       +8     
==========================================
+ Hits        16405    16454      +49     
+ Misses       1609     1600       -9     
  Partials      702      702              

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