Allow user to set a certain commit body when enabling auto-merge on a PR

[HonkingGoose]

What would you like Renovate to be able to do?

Summary: a way to tell Renovate to set a certain commit body when enabling auto-merge on a PR.

Relevant quote from @AlexanderWirthSAP:

So I don't want to remove information from the prBody, I am looking for a way to tell Renovate to set a certain commit body when enabling auto-merge on a PR. Based on your replies, it seems that this is currently not possible. I think it's a reasonable feature request though, given my example of the current behavior above. All I would need is some new setting like for example "automergeCommitBody" that lets me control the commit body that Renovate should use when enabling auto-merge.

Current behavior

commit 8524bffde3aeb3144d720d950f3bc1317e59ea41 (HEAD -> master, origin/master)
Author: ospo-renovate[bot] <84706+ospo-renovate[bot]@users.noreply.github.wdf.sap.corp>
Date:   Thu Apr 6 14:16:21 2023 +0200

    Update dependency com.sap.sgs:security-patched-spring-boot-starter to v3.0.5-sap-01 (#38)

    This PR contains the following updates:

    | Package | Type | Update | Change |
    |---|---|---|---|
    | [com.sap.sgs:security-patched-spring-boot-starter](https://spring.io/projects/spring-boot) ([source](https://github.wdf.sap.corp/Phosphor/security-patched-spring-boot-30x)) | parent | patch | `3.0.4-sap-01` -> `3.0.5-sap-01` |

    ---

    ### Release Notes

    <details>
    <summary>Phosphor/security-patched-spring-boot-30x</summary>

    ### [`v3.0.5-sap-01`](https://github.wdf.sap.corp/Phosphor/security-patched-spring-boot-30x/blob/HEAD/CHANGELOG.md#&#8203;305-sap-01---2023-03-23)

    ##### Update

    -   Updated to `org.springframework.boot:spring-boot-starter-parent:3.0.5`
        and `org.springframework.boot:spring-boot-dependencies:3.0.5`.

    ##### Security

    -   Updated `org.codehaus.jettison:jettison` from `1.5.2` to `1.5.4`
        to fix [CVE-2023-1436](https://nvd.nist.gov/vuln/detail/CVE-2023-1436).

    ### [`v3.0.4-sap-02`](https://github.wdf.sap.corp/Phosphor/security-patched-spring-boot-30x/blob/HEAD/CHANGELOG.md#&#8203;304-sap-02---2023-03-10)

    ##### Security

    -   Updated `org.wildfly.security:wildfly-elytron` from `1.20.1.Final` to `1.20.3.Final` which inturn updates `org.apache.sshd:sshd-common` from `2.7.0` to `2.9.2`
        to fix [CVE-2022-45047](https://nvd.nist.gov/vuln/detail/CVE-2022-45047).

    </details>

    ---

    ### Configuration

    📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

    🚦 **Automerge**: Enabled.

    ♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

    🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

    ---

     - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

    ---

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Desired behavior

commit 8524bffde3aeb3144d720d950f3bc1317e59ea41 (HEAD -> master, origin/master)
Author: ospo-renovate[bot] <84706+ospo-renovate[bot]@users.noreply.github.wdf.sap.corp>
Date:   Thu Apr 6 14:16:21 2023 +0200

    Update dependency com.sap.sgs:security-patched-spring-boot-starter to v3.0.5-sap-01 (#38)

If you have any ideas on how this should be implemented, please tell us here.

@viceice said: ¹

that sounds good to me. it should be undefined by default to not break existing users.

Is this a feature you are interested in implementing yourself?

No

Footnotes

1. https://github.com/renovatebot/renovate/discussions/21369#discussioncomment-5555238 ↩

[viceice]

i think this is GitHub specific