Don’t show source project name when user does not have access

See merge request !2081

Closes #29660

Merge request !7163 caused an error message to appear on the Merge
Request form when it was first loaded by the user. This commit fixes
that regression bug.

Steps to reproduce:

1. Start with a repo with an image
2. Add a commit to delete the image
3. Add another commit to replace the image with another image

In a diff comparison, we really just compare about what the image was before the diff, not
the direct parent of the last commit. This MR fixes that.

Closes #3893, gitlab-org/gitlab-ee#678
Signed-off-by: Rémy Coutable <remy@rymai.me>

If source branch or target branch doesn't exist in the project, display
form validation errors. Previously, this caused a 500 error code.

Test was not waiting for the page to be fully loaded

Fixes #15591.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Closes #14875