If you're going to build, say, a directory structure where a directory is named for a commit in a Git repository, and you want it to be short enough to make your eyes not bleed, but long enough that the chance of it colliding would be negligible, how much of the SHA substring is generally required? Let's say I want to uniquely identify this change: https://github.com/wycats/handlebars.js/commit/e6
