Add --path filter to darc vmr diff

[Copilot]

darc vmr diff always diffed the entire repository with no way to scope it to specific paths.

Usage

# Diff only src/libraries
darc vmr diff C:\Path\VMR..https://github.com/dotnet/runtime:main --path src/libraries

# Diff multiple paths
darc vmr diff C:\Path\VMR..https://github.com/dotnet/runtime:main --path src/libraries --path src/coreclr

# Also works with --name-only
darc vmr diff --name-only --path src/libraries

#5572

[premun]

Tested locally, works

[Copilot]

Pull request overview

Adds a --path filter to darc vmr diff to allow scoping diffs to one or more subpaths, improving usability when working with large repos/VMRs.

Changes:

• Introduced --path (multi-valued) option on darc vmr diff.
• Full diff mode: prepends user-provided pathspecs to the existing exclusion filter list and normalizes path separators.
• Name-only mode: post-filters computed differences by path prefix and normalizes path separators; updated docs with examples.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/Microsoft.DotNet.Darc/Darc/Options/VirtualMonoRepo/VmrDiffOptions.cs	Adds the --path CLI option for scoping diffs.
src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/VmrDiffOperation.cs	Applies --path filtering in both full diff and name-only diff modes.
docs/Darc.md	Documents --path usage and provides examples.

[Copilot]

--path normalization differs between full diff and name-only modes. Here only backslashes are replaced, but leading '/', './', or '' aren’t trimmed like they are in FileTreeDiffAsync, which can make --path /src/foo behave differently depending on --name-only. Consider centralizing path normalization (trim leading separators/"./", replace '\' with '/', and ignore/throw on empty/whitespace entries) and using it in both modes before building allFilters.

[Copilot]

The name-only --path filter uses StartsWith, which can over-match sibling paths (e.g., --path src/lib would also include src/libraries). If the intention is to treat --path as a directory/file boundary (like git pathspecs), consider matching either exact path or prefix followed by '/' (and normalize any trailing slashes). Also consider whether OrdinalIgnoreCase is desired on case-sensitive platforms; using an OS-dependent comparison would better mirror git behavior.

Suggested change

	fileDifferences = fileDifferences
	.Where(kv => normalizedPaths.Any(p => kv.Key.StartsWith(p, StringComparison.OrdinalIgnoreCase)))
	var pathComparison = OperatingSystem.IsWindows()
	? StringComparison.OrdinalIgnoreCase
	: StringComparison.Ordinal;
	bool PathMatchesFilter(string filePath, string filterPath)
	{
	// Both filePath and filterPath are expected to be normalized to use '/' and start with '/'
	if (filePath.Equals(filterPath, pathComparison))
	{
	return true;
	}
	if (!filterPath.EndsWith("/", StringComparison.Ordinal))
	{
	filterPath += "/";
	}
	return filePath.StartsWith(filterPath, pathComparison);
	}
	fileDifferences = fileDifferences
	.Where(kv => normalizedPaths.Any(p => PathMatchesFilter(kv.Key, p)))

[dkurepa]

so we essentially do a full diff and filter it?
Prob doesn't really matter but in theory we could not search the files that are not on the provided paths too

[premun]

Right. Yeah, that would make it run much faster. @copilot can you make this change?

[dkurepa]

rip