Unique abbreviated Commit IDs

[ajoberstar]

A unique abbreviated commit ID can be retrieved with ObjectReader. That should be the default if no arguments are provided:

getAbbreviatedId() should return a 7 character or longer (as needed to be unique)
getAbbreviatedId(int) should retain the current behavior of just giving you the exact length you asked for

However, it may make sense to deprecate the int variant.