Show first 7 characters of hash

Since the actual value of the hash isn't useful after some amount of bits, I
looked up how many characters GitHub uses in their UI. It's 7. If that's good
enough for them I think it's good enough for us too.

Test Plan:
 * Edited a line
 * Reloaded window,
 * Saw only 7 characters in hash
 * Saw only 7 hyphens in my edited line