Increase short SHA length to 7 characters (#11492)

This PR increases the length of a shortened Git SHA from 6 to 7
characters.

This matches what GitHub uses.

I also took the opportunity to factor out a common method for computing
a short SHA so that we have a single source of truth for the length that
we're using.

Release Notes:

- Increased the short commit SHA length used by git blame from 6 to 7
characters.