User shortName instead of substring on sha copy

[mczepiel]

Nothing functionally different, just using what the commit offers. In the process of some other changes locally.

[mczepiel]

FWIW it looks like shortName will always return a SHA, so this is functionally equivalent to the manual substring:

- (NSString *) shortName
{
    return self.gtCommit.shortSHA;
}

So this should be fine so long as we're alwasy dealing with commits and not refs, which have a different shortName behavior:
// shortName a more user friendly version of the refName, "master" or a short SHA