import-tar: add --strip-components

[ThomasWaldmann]

Fixes #6461.

Add --strip-components to borg import-tar, with the same semantics as the existing option of borg extract / borg export-tar: remove the given number of leading path elements from the tar member names when creating the archive items; members whose path has fewer or equally many elements are silently skipped.

Details:

• The option definition is factored out of define_exclude_and_patterns into a shared define_strip_components helper, so all commands use the identical definition/help text (no behavior change for extract/export-tar/mount).
• The skip decision happens in the _import_tar member loop, counting components of the normalized stored path (make_path_safe(posixpath.normpath(name))), consistent with how TarfileObjectProcessors computes item paths.
• The path rewrite happens in TarfileObjectProcessors.create_helper, so it also covers BORG-format tars, where items are restored from the BORG.item.meta pax header and tarinfo.name is not consulted.
• Hard link targets are stripped the same way, so LNKTYPE members keep matching the stripped paths remembered in the HardLinkManager and still reuse their target's chunks; a hard link whose target was skipped is skipped, too.
• Symbolic link targets are left unchanged (they are content, matching GNU tar behavior).

Tests: basic stripping incl. ./-prefixed member names and the strip-everything → empty archive case, hardlink/symlink behavior, and a BORG-format round-trip exercising the pax fast path.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.25%. Comparing base (6489248) to head (219d145).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10220      +/-   ##
==========================================
+ Coverage   88.23%   88.25%   +0.01%     
==========================================
  Files         103      103              
  Lines       18876    18898      +22     
  Branches     2933     2938       +5     
==========================================
+ Hits        16656    16678      +22     
  Misses       1544     1544              
  Partials      676      676              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.