ExportChanges: use POSIX / Unix conventions for Tar operations

[thaJeztah]

The Change.Path field holds a local path, but it was used to set the Tar.Name field.

Convert the path to a POSIX / Unix path before setting. Also explicitly convert the archive-path to a POSIX path when calling addTarFile, and explicitly strip a leading / (if present), instead of the first character.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.07%. Comparing base (2cd730e) to head (d108a5d).
⚠️ Report is 41 commits behind head on main.

Files with missing lines	Patch %	Lines
changes.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   66.35%   65.07%   -1.28%     
==========================================
  Files          42       42              
  Lines        2027     2033       +6     
==========================================
- Hits         1345     1323      -22     
- Misses        497      535      +38     
+ Partials      185      175      -10     

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[thaJeztah]

GoDoc says;

// ExportChanges produces an Archive from the provided changes, relative to dir.

So ... relative to dir, but not sure if that should also mean "outside of dir", but that's something for a follow-up.

[thaJeztah]

Moby code uses it like this, so yeah, probably expected to be within dir

	parentFs, err := driver.Get(parent, "")
	if err != nil {
		return nil, err
	}
	defer driver.Put(parent)

	changes, err := archive.ChangesDirs(layerFs, parentFs)
	if err != nil {
		return nil, err
	}

	tarArchive, err := archive.ExportChanges(layerFs, changes, gdw.IDMap)
	if err != nil {
		return nil, err
	}

[Copilot]

Pull request overview

This PR fixes tar archive path handling in ExportChanges by ensuring paths written into tar headers use POSIX (forward-slash) conventions and don’t start with an absolute /, which can be problematic for tar consumers and is especially incorrect for Windows-style paths.

Changes:

• Normalize whiteout tar header names using filepath.ToSlash and strings.TrimPrefix(..., "/") instead of slicing off the first byte.
• Compute a POSIX archivePath for addTarFile to ensure hardlink bookkeeping (SeenFiles / Linkname) uses consistent forward-slash paths.
• Replace ambiguous path variable naming with clearer srcPath and archivePath.

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