fix(dist-git): use first-parent for snapshot-time commit resolution

[Tonisal-byte]

Problem

GetCommitHashBeforeDate walks all parents when resolving the upstream commit
via snapshot time. This can resolve to a commit from a merged-in side branch
(e.g. an f44 commit merged into f43), causing "import-commit not found"
errors during synthetic history generation because the first-parent walk
can't reach the import-commit from a side-branch commit.

Fix

Add --first-parent to the git rev-list command so the resolved commit is
always on the target branch's mainline. The merge commit that incorporated
the side branch is the correct anchor — it has the same tree content and
sits on the first-parent chain where the synthetic history walk expects it.

[Copilot]

Pull request overview

This PR adjusts dist-git snapshot commit resolution so timestamp-based upstream commit selection stays on the target branch’s first-parent history, avoiding side-branch commits that synthetic history generation cannot reach.

Changes:

• Adds --first-parent to git rev-list in GetCommitHashBeforeDate.
• Expands the inline comment to explain why first-parent traversal is required for snapshot-time resolution.