Fix handling fully-reverted files during rebase codeflow

[premun]

Rebase conflicts on all reverted files and we are able to resolve them using the crossing flow.

• The reverts happen if there are 2 flows in the same direction with a conflict.
• For 2 flows in the same direction, there's no crossing flow.
• If we detect recreation of previous flows, we're technically unwinding the latest flow (or more) and recreating them. But that means, the latest flow in the same direction is now a crossing flow.
• When we detect this, we set the lastFlows.CrossingFlow to the last flow in the same direction and the resolver of the gradual changes / reverted files then correctly fixes the files.

#5541

[Copilot]

Pull request overview

This PR fixes an issue where rebase codeflow encounters conflicts on reverted files when there are two flows in the same direction. The solution detects when previous flows are being recreated and treats the last same-direction flow as a crossing flow, enabling proper conflict resolution for reverted files.

Key Changes:

• Implements crossing flow detection for recreated flows in both forward and backward flow scenarios
• Adds file revert resolution logic for files that were added then removed during rebase
• Enhances conflict handling to commit changes when no conflicts exist during backflow

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrForwardFlower.cs	Adds crossing flow detection when previous flows are recreated
src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrBackflower.cs	Implements crossing flow detection and conditional commit logic for conflict-free backflows
src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrCodeflower.cs	Refactors reverted file handling with new FileToBeRemovedContent constant and commit logic
src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/CodeFlowConflictResolver.cs	Adds TryRevertingAddedFile method and improves conflict abort logic
src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/ForwardFlowConflictResolver.cs	Integrates file revert resolution for forward flows
src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/BackflowConflictResolver.cs	Adds file revert resolution for backflows
src/Microsoft.DotNet.Darc/DarcLib/Models/VirtualMonoRepo/CodeFlowResult.cs	Adds RecreatedPreviousFlows property to track flow recreation
test/Microsoft.DotNet.DarcLib.Codeflow.Tests/TwoWayCodeflowTests.cs	Updates OutOfOrderMergesWithConflictsTest to test rebase-only scenarios with new flow verification
test/Microsoft.DotNet.DarcLib.Codeflow.Tests/ForwardFlowTests.cs	Removes test ignore and fixes assertion for file content expectations
test/Microsoft.DotNet.DarcLib.Codeflow.Tests/BackflowTests.cs	Adds comprehensive BackflowWithRevertsAndConflictsTest mirroring forward flow test
test/Microsoft.DotNet.DarcLib.Codeflow.Tests/Helpers/GitOperationsHelper.cs	Changes CreateBranch to use -B flag and removes --ff-only from merge
eng/Create-CodeflowGraphVizGraph.ps1	Adds --first-parent flag to git blame commands

[premun]

@dkurepa there is one failing test ForwardFlowWithConflictsDeepInPastTest which is hitting an NRE here:

arcade-services/src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/ForwardFlowConflictResolver.cs

Line 128 in 297d9f9

? lastFlows.LastBackFlow!.VmrSha

Do you know what's the thinking behind that ! and whether it shouldn't be something more defensive?

[premun]

@dkurepa let me know if 99541bb is a good fix

[dkurepa]

@dkurepa let me know if 99541bb is a good fix

I think so, yes, it's also an edge case that will only happen when we add a new VMR repo too