Change message on Special:Wantedfiles based on if foreign repo on.

If foreign repos are off, no need to warn about false positives.

I mostly want this change for an upcoming change I'm making in
GlobalUsage - I660a61327cdb528

Change-Id: Ibcf63d48c50523722ce8384fa6b4b1f68524482c
Merge "Change message on Special:Wantedfiles based on if foreign repo on."