create: add --exclude-dataless to skip cloud files not materialized locally

[ThomasWaldmann]

Ports #9755 to master.

macOS flags files whose content lives in cloud storage (e.g. iCloud Drive) and is not present locally with SF_DATALESS. Reading such a file triggers downloading its content.

This adds --exclude-dataless to borg create: the flags are checked right after stat() and before any open(), so excluded files/dirs never get materialized. Skipped paths are reported with the usual x status, analogous to --exclude-nodump.

Adaptation for master

In master, archiver.py was split into archiver/ and the nodump exclusion moved into maybe_exclude_by_attr() in archive.py — which runs after the file is opened. Opening a dataless file triggers the cloud download, so SF_DATALESS cannot be handled there. The check is placed in _rec_walk() right after stat() and before the file/dir branch, matching the spirit of the 1.4-maint PR.

Notes

• stat.SF_DATALESS only exists from Python 3.13 on; falls back to 0x40000000 from macOS' sys/stat.h.
• The check covers directories too (they can also be dataless), skipping before OsOpen / recursion.
• Test fakes get_flags via monkeypatch since SF_DATALESS cannot be set from userspace.
• Fish shell completion updated; bash/zsh completions don't exist in master.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.88%. Comparing base (0b10329) to head (f484741).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9756      +/-   ##
==========================================
+ Coverage   84.86%   84.88%   +0.01%     
==========================================
  Files          92       92              
  Lines       14970    14979       +9     
  Branches     2234     2236       +2     
==========================================
+ Hits        12705    12715      +10     
  Misses       1571     1571              
+ Partials      694      693       -1     

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