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

test/init.bats

@@ -21,7 +21,7 @@ load test_helper
   root="$(cd $BATS_TEST_DIRNAME/.. && pwd)"
   run rbenv-init - bash
   assert_success
-  assert_line "source '${root}/libexec/../completions/rbenv.bash'"
+  assert_line "source '${root}/test/../libexec/../completions/rbenv.bash'"
 }
 
 @test "detect parent shell" {
@@ -35,7 +35,7 @@ load test_helper
   root="$(cd $BATS_TEST_DIRNAME/.. && pwd)"
   run rbenv-init - fish
   assert_success
-  assert_line ". '${root}/libexec/../completions/rbenv.fish'"
+  assert_line ". '${root}/test/../libexec/../completions/rbenv.fish'"
 }
 
 @test "fish instructions" {