Proposal: Allow Customization of Git Merge Commit Message Template

Hello Git mailing list,

I’d like to propose a feature to allow users to modify the default merge commit message template generated by `git merge`. Currently, when running:

``` git merge --no-ff upstream/main ```

Git produces a message like: ```sh Merge remote-tracking branch 'upstream/main' into my-branch ```

While informative, this message can be verbose and sometimes makes branch names difficult to fit within the recommended 52-character first line. Personally, I find a simplified format like this more concise and effective: ```sh Merge upstream/main into my-branch ```

Proposal: Introduce an option (e.g., a configuration setting or CLI flag) that allows users to specify a custom merge commit message template. For example:

- A new configuration option:
  ```sh
  [merge]
      messageTemplate = "Merge %s into %s"
  ```
  Here, `%s` placeholders could represent the source and destination branches, respectively.

- Alternatively, a command-line flag:
  ```sh
  git merge --message-template="Merge %s into %s" upstream/main
  ```

Looking forward to your thoughts.

/k