Eliminate unneeded calls to Repository#blob_at when listing commits with no path

What does this MR do?

It rearranges the conditions in CommitsHelper#link_to_browse_code to avoid calling Repository#blob_at needlessly when viewing a commits page without a path.

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

It can speed things a little bit.

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together