Add more useful Open Graph metadata for commit and file URLs:
- Set `og:title` to the page title, which is a concise summary in both
cases (`<commit message> · <commit hash>` and `<filename> at <branch>`,
respectively)
- Set `og:description` to the commit message body, if available
- Set `og:url` to the relevant URLs instead of the repo URL
Also move the relevant meta tags into a separate template as they now
take up the majority of the base head template.
