It doesn't try to chdir into RBENV_ROOT anymore because that might be
a location of an unrelated rbenv install that might have a different
version than the current one that is installed e.g. via a package
manager such as Homebrew.
Now just tries the repo where the source files (`libexec/*`) are
located, and if that isn't a valid rbenv repo, bail out early.
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.
