When `rbenv --version` is called, this now happens:
1. It changes into the directory where `libexec/rbenv--version` resides
and checks if it's a checkout of the rbenv repo (as opposed to
Homebrew checkout or something else). Then it reads the git revision.
2. If that failed, change to `$RBENV_ROOT` directory and repeat step 1.
dcca61c0bc