The GitCommitInfo MSBuild task was failing with an error when encountering "grafted" repository states, which can occur when copilot or other git operations create grafted repositories.

Problem:
When git log -n 1 --pretty=%D HEAD returns output starting with "grafted" (e.g., "grafted, HEAD"), the current parsing logic doesn't recognize this format and throws an error:

error : Unable to parse branch name from: grafted, HEAD

This causes the build to fail instead of handling the edge case gracefully.

Solution:
Added support for detecting "grafted" repository states in both the C# implementation (GitCommitInfo.cs) and the corresponding shell script (get-git-branch.sh). When a grafted repository is detected:

• Logs a warning instead of an error
• Sets the branch name to "unknown" (consistent with other ambiguous cases)
• Allows the build to continue successfully

Changes:

• GitCommitInfo.cs: Added new condition to handle branchFull.StartsWith("grafted")
• get-git-branch.sh: Added corresponding logic to maintain sync between implementations

Impact:

• Builds now succeed with a warning instead of failing when encountering grafted repositories
• All existing branch detection patterns continue to work unchanged
• Minimal code changes (7 lines added total)

Fixes #10210.

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