The abbreviated commit hash we show in the shell and the about
dialog is currently just the last 7 characters of 'git describe',
based on the assumption that abbreviated hashes are always 7-digits
long.  When the hash is longer than that, we're just showing a
nonsense commit.

This was never a good idea, since users can override this, and
since disambiguation can result in longer hashes, but since git
2.11, the default abbreviated hash length is determined based on
the size of the repository, which currently results in 10 digits
for us.

Let's just do it right.