RebaseArchiveEntries: use POSIX / Unix paths

[thaJeztah]

This is mostly defense-in-depth; RebaseArchiveEntries is manipulating the Tar headers, which use POSIX / Unix paths; convert the given paths on the way in.

[Copilot]

Pull request overview

This PR hardens RebaseArchiveEntries by normalizing oldBase/newBase to POSIX-style paths before rewriting tar headers, aligning with tar’s expectation of /-separated entry names.

Changes:

• Normalize oldBase and newBase using filepath.ToSlash() before rebasing tar header names.
• Treat oldBase == "/" as a special case (root) after normalization.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.33%. Comparing base (2cd730e) to head (ef0bff1).
⚠️ Report is 41 commits behind head on main.

Files with missing lines	Patch %	Lines
copy.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   66.35%   65.33%   -1.02%     
==========================================
  Files          42       42              
  Lines        2027     2034       +7     
==========================================
- Hits         1345     1329      -16     
- Misses        497      532      +35     
+ Partials      185      173      -12     

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[thaJeztah]

Heh; new test found a bug; I'll remove that test-case, and do a follow-up;

=== RUN   TestRebaseArchiveEntriesPlatformPaths/hardlink_rebase_from_root
    copy_test.go:104: assertion failed: prefix/link (tc.wantName string) != prefixlink (got.Name string)
    copy_test.go:105: assertion failed: prefix/target (tc.wantLinkName string) != prefixtarget (got.Linkname string)