Add the darc vmr diff --name-only option

[premun]

Adds a new option that only lists the differing file names

dotnet/source-build#5201

[Copilot]

Pull Request Overview

Adds a new --name-only flag to the darc vmr diff command so users can list just the changed file names instead of full diffs.

• Defines a NameOnly boolean option in the command-line options class.
• Updates the diff operation to branch on options.NameOnly, outputting either file names or full diffs.
• Wraps the diff logic in a try/finally to ensure the temporary directory is always cleaned up.

Reviewed Changes

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

File	Description
src/Microsoft.DotNet.Darc/Darc/Options/VirtualMonoRepo/VmrDiffOptions.cs	Add --name-only option to the VMR diff verb
src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/VmrDiffOperation.cs	Implement name-only output path and console logic, adjust diff and cleanup flow

Comments suppressed due to low confidence (2)

src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/VmrDiffOperation.cs:290

• Add unit or integration tests for the new --name-only flag, covering both console output and writing to options.OutputPath, to verify correct behavior.

if (options.NameOnly)

src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/VmrDiffOperation.cs:288

• The removal of the if (!string.IsNullOrEmpty(tmpDirectory)) guard causes full diffs to always be printed to stdout even when an output path is provided. Reintroduce an output-path check in the regular diff branch to correctly save diffs to files when options.OutputPath is set.

try