Display source branch name in the Merge Request edit form

Description

Display source branch name in the Merge Request edit form

Proposal

When editing a Merge Request, it's sometimes necessary to determine the source branch name to discern further information to flesh out the description or title.

Let's display the source branch name in the Edit form to expose this information here as well as in the show view.

Links / references

Use cases

Who is this for? Provide one or more use cases.

Project Maintainers aiming to provide more information in Merge Request description. When editing they may need the source branch info so the MR's associated branch can be checked out.