Store: small refactoring to lift the merge code

Very small refactoring to extract the code that runs executes the merge outside a sub function, making the set_head more readable.

This is a trivial change but it must be reviewed carefully.

Manually testing the MR

Green CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR