Implements support for diffing against the last backflow when darc vmr diff is called from a product repository (not a VMR).
The same way we added darc vmr diff called from VMR and diffing based on source-manifest.json, we can now call it from the repo and it will use the <Source> tag.

What this changes

Previously, darc vmr diff could only be used from within a VMR directory when no repository arguments were provided. This PR adds support for calling it from a product repository that contains eng/Version.Details.xml.

When called from a product repository:

• The command reads the <Source> element from eng/Version.Details.xml
• Extracts the VMR URI and SHA to diff against
• Performs the diff between the current product repo and the specified VMR state

Examples

# From a product repository with eng/Version.Details.xml
cd /path/to/my-product-repo
darc vmr diff --name-only

# Output:
# Diffing against VMR https://github.com/dotnet/dotnet / abc123def456
# --------------------------------------------------------------------------------
# + NewFile.cs
# - RemovedFile.cs
# * ModifiedFile.cs

# Full diff mode also works
darc vmr diff

Error handling

The command provides clear error messages for various scenarios:

• No git repo: "can only be used from within a git repository directory"
• Not VMR and no Version.Details.xml: "can only be used from within a VMR directory or a product repository with eng/Version.Details.xml"
• No Source element: "No element found in Version.Details.xml. Cannot determine VMR to diff against."

Backward compatibility

Existing VMR behavior is unchanged - when called from a VMR directory, it continues to diff against all repositories in source-manifest.json as before.

Fixes #5212.

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