Stop using abs_dirname() in rbenv-init

It's slow and not necessary since we expect `$0` to already be expanded.

In tests this change forces us to deal with some relative paths, but
it's not a big deal. The `rbenv init -` output in the most common case
will be the same as before:

    source '/home/myuser/.rbenv/libexec/../completions/rbenv.bash'

libexec/rbenv-init

@@ -28,31 +28,7 @@ if [ -z "$shell" ]; then
   shell="$(basename "${shell:-$SHELL}")"
 fi
 
-READLINK=$(type -p greadlink readlink | head -1)
-if [ -z "$READLINK" ]; then
-  echo "rbenv: cannot find readlink - are you missing GNU coreutils?" >&2
-  exit 1
-fi
-
-resolve_link() {
-  $READLINK "$1"
-}
-
-abs_dirname() {
-  local cwd="$(pwd)"
-  local path="$1"
-
-  while [ -n "$path" ]; do
-    cd "${path%/*}"
-    local name="${path##*/}"
-    path="$(resolve_link "$name" || true)"
-  done
-
-  pwd
-  cd "$cwd"
-}
-
-root="$(abs_dirname "$0")/.."
+root="${0%/*}/.."
 
 if [ -z "$print" ]; then
   case "$shell" in