New `dt` tool: `merge-commit`

In order to make sure a cherry-picked commit is not garbage collected by Github, we perform a merge-commit following the instructions here: https://github.com/flutter/devtools/blob/master/tool/RELEASE_INSTRUCTIONS.md#5-create-the-merge-commit-in-the-flutterdevtools-repo

In some scenarios (e.g. https://github.com/flutter/devtools/pull/9722) this can lead to a diff with file changes.

It's more reliable to follow the steps here: https://github.com/dart-lang/sdk/blob/main/docs/Cherry-picks-to-a-release-channel.md#cherry-picking-a-commit-in-a-dependency

In addition, we can wrap those steps in a `dt` tool (e.g. `merge-commit`) to make it even easier.

Start by reading the linked Flutter DevTools release instructions and the Dart SDK cherry-pick guide, focusing on the merge-commit steps and the cases where they produce file changes. Then locate the existing `dt` tool entry point and determine how the `merge-commit` command should wrap that workflow; done means the command reliably performs the documented steps without unintended file changes.

索引モデルが issue の本文から書いたものです。