Use a better meta description for repository pages (BB-2698)

I'm sharing links to BitBucket repositories & files fairly often on Google+. Unfortunately, the description that Google+ detects is always the same and the generic "This user is sharing code with you [...]".

It would be so much more useful if you could use the repository's description (when available) instead of the generic one. This would also make the BB pags much more SEO friendly, as they include relevant information in their meta tags.