Clarify the type of diff shown in the merge request

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Proposal

When creating an MR, we show a diff to the user. As per @patrickbajao we show a merge base diff on the new merge request form since we don't do a straight comparison of source and target branch.

This might be misleading to the user as the user might expect this would be a diff between the source branch and the target branch.

Related ticket:

• https://gitlab.zendesk.com/agent/tickets/403608 (internal)