[QA] Fix 2 of 3 problems with the push file size tests

What does this MR do?

This stops the tests in push_over_http_file_size_spec.rb from trying to re-create master as a new branch. And it should expect a Git error to be raised when the file it too large.

There's still one more problem due to not having admin access on staging, but that will require coordinated changes to CE and gitlab-qa, so I'll do that separately.

What are the relevant issue numbers?

gitlab-org/quality/staging#37 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer