Check for a single line number instead of arbitrary text

What does this MR do?

Rel: #241753 (closed)

Changes the way the page-load detection works for page/merge_request/show.rb.

Previously, the check would be for "No newline at end of file." The problem with that is not all code abides by this practice. Some code files do not have newlines at EOF, others do. Because of this variability, I think it would be safer to check for a line number. Additionally, I think the line number is a good check since you cannot leave a comment on code with no changed lines.

Note: I'm aware that this might not fix the specific problem of the test. This is more-so a correction to an issue I've found in our source code, with the hopes that it also fixes the issue 😉

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content