Add link to fixed SHA version on blob.

Fixes AMR: http://feedback.gitlab.com/forums/176466-general/suggestions/5765123-shortcut-key-to-go-a-static-canonical-version-of-t

Screenshot:

![screenshot from 2014-08-08 16 04 54 fixed sha url blob](https://cloud.githubusercontent.com/assets/1429315/3878393/d90610ea-216e-11e4-960b-7b0c422d3b23.png)

The button will disappear when already on a SHA URL.