Don't add the new header if the review request doesn't have a repository.
Don't add the new header if the review request doesn't have a repository.

The recent change to add a X-ReviewRequest-Repository header for e-mails
broke for review requests without a repository. It's now conditionally
added.

1tush/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,davidt/reviewboard,KnowNo/reviewboard,KnowNo/reviewboard,KnowNo/reviewboard,beol/reviewboard,brennie/reviewboard,chipx86/reviewboard,1tush/reviewboard,sgallagher/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,custode/reviewboard,sgallagher/reviewboard,custode/reviewboard,chipx86/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,davidt/reviewboard,beol/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,beol/reviewboard,sgallagher/reviewboard,1tush/reviewboard,chipx86/reviewboard,KnowNo/reviewboard,davidt/reviewboard,1tush/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,beol/reviewboard,reviewboard/reviewboard,davidt/reviewboard