fix: prevent duplicate content in merge-previous-releases auto-merge

[sleepytanya]

When merging older RCs into a new RC, git's auto-merge can create duplicate content (e.g., duplicate locale keys) when both branches have identical changes. This happens because git doesn't recognize the changes are the same and tries to combine them.

Fix: use --no-commit --no-ff, then reset all files to destination before committing. This preserves merge history while keeping destination content.

Fixes: https://consensyssoftware.atlassian.net/browse/MCRM-143

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Note

Medium Risk
Changes release-branch automation that pushes merge commits; behavior now discards any post-merge tree differences from older RCs in favor of the new branch, which is intentional but affects production release flows.

Overview
Fixes merge-previous-releases so merging an older RC into a new one no longer duplicates content (e.g. repeated locale keys) when both branches contain the same edits—Git was combining identical changes instead of treating them as one.

merge_with_favor_destination no longer relies on -X ours and per-file conflict resolution. It runs git merge --no-commit --no-ff, then git checkout HEAD -- . to force the working tree to match the new RC, stages (still excluding github-tools), and commits the merge. Merge history is kept; file content always matches the destination branch.

The script header comments and CHANGELOG document the new behavior and link PR #283.

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[sleepytanya]

@Gudahtt
Two edge cases: a) if old RC has content not yet in main, it gets discarded; b) if destination deleted file that source has, it gets un-deleted. Not sure if this can happen in real life?