Hooks: Add Source Branch Name to Merge Events

[nopcoder]

Currently, the merge pre/post events provide information about the destination branch and the source ref as the commit that was used to merge into the branch.

This proposal suggests adding the name of the source branch as part of the merge event data.

Identifying the source branch name during a merge operation offers several benefits:

• Enforce Branch-Specific Preferences: Different branches might have unique merging requirements or preferences. Knowing the source branch allows for tailored actions based on its origin.
• Capture Original Request Information: The source branch name provides valuable context about the original merge request, enhancing traceability and understanding of the operation.