Fix invoking rbenv update from rbenv-sh-update

It's not a good idea to invoke `rbenv-update` executable with absolute
path because that bypasses rbenv. Instead, `command rbenv update` does
the trick and ensures that RBENV_ROOT will always be available.

bin/rbenv-sh-update

@@ -6,13 +6,13 @@ shell="$(basename "${RBENV_SHELL:-$SHELL}")"
 
 case "$shell" in
 fish )
-  echo -n "$(which rbenv-update); and "
+  echo -n "command rbenv update; and "
   echo -n 'echo -e "\033[1;32mreloading rbenv\033[0m"; and '
   echo -n '. (rbenv init - | psub); and '
   echo -n 'echo -e " \033[1;32m|\033[0m  done"'
   ;;
 * )
-  echo -n "$(which rbenv-update) && "
+  echo -n "command rbenv update && "
   echo -n 'echo -e "\033[1;32mreloading rbenv\033[0m" && '
   echo -n 'eval "$(rbenv init -)" && '
   echo -n 'echo -e " \033[1;32m|\033[0m  done"'

bin/rbenv-update

@@ -25,8 +25,7 @@ rbenv_update() {
 cd "$(dirname $(which rbenv))"
 rbenv_update rbenv
 
-cd "${RBENV_ROOT:-$(rbenv root)}"
-for plugin in plugins/*; do
+for plugin in "$RBENV_ROOT"/plugins/*; do
   pushd $plugin >/dev/null
   rbenv_update `basename $plugin`
   popd >/dev/null