Show HEAD comparison on merge request creation page

Problem to solve

On the MR creation page we only show base comparison of changes from source branch to target branch while on the MR page, we show a straight comparison of changes from source branch to target branch to show how the changes will look like when merged (only if there are no conflicts). This can be confusing to users because of the difference of output.

Proposal

Based on #371018 (comment 1450455138):

Change the comparison on the MR creation page to:

1. Compare source branch to target branch as if it'll be merged (just like HEAD diff on MR changes tab) if possible.
2. If not possible (e.g. conflict), revert to base comparison.
3. Track users using the "Changes" tab on the MR creation page.