1. The `og:description` should be "a one to two sentence description of
your object"
* It shouldn't output all the user inputted content -- it would be
pretty huge.
* Maybe it only needs at most 300 bytes.
2. Do not render commit message as HTML
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.
