New `dt` tool: `merge-commit`

[elliette]

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. #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.