Fix traversing PATH for bash < 4.4

Bash versions before 4.4 did not support the `-d` nor `-t` flags for readarray:
https://lists.gnu.org/archive/html/info-gnu/2016-09/msg00012.html

This switches away from readarray/"read" completely in favor of traversing PATH
by using string substitution.

libexec/rbenv-help

@@ -154,25 +154,27 @@ print_usage() {
 if [ "$1" = "--complete-commands" ]; then
   command_prefix="${2:-}"
   seen=()
-  if [ "$(type -t readarray)" = "builtin" ]; then
-    readarray -d : -t paths < <(printf "%s" "$PATH")
-  else
-    # bash 3.x compatibility
-    IFS=: read -r -a paths <<<"$PATH" || true
-  fi
   shopt -s nullglob
-  for path in "${paths[@]}"; do
-    for command in "${path}/rbenv-${command_prefix}"*; do
-      command_name="${command##*/}"
-      command_name="${command_name#rbenv-}"
-      command_name="${command_name#sh-}"
-      [[ " ${seen[*]} " != *" ${command_name} "* ]] || continue
-      seen+=("$command_name")
-      summary=""
-      eval "$(extract_initial_comment_block < "$command" | collect_documentation)"
-      [ -n "$summary" ] || continue
-      printf "%s:%s\n" "$command_name" "$summary"
-    done
+  PATH_remain="$PATH"
+  # traverse PATH to find "rbenv-" prefixed commands
+  while true; do
+    path="${PATH_remain%%:*}"
+    if [ -n "$path" ]; then
+      for rbenv_command in "${path}/rbenv-"*; do
+        command_name="${rbenv_command##*/}"
+        command_name="${command_name#rbenv-}"
+        command_name="${command_name#sh-}"
+        [[ $command_name == "${command_prefix}"* ]] || continue
+        [[ " ${seen[*]} " != *" ${command_name} "* ]] || continue
+        seen+=("$command_name")
+        summary=""
+        eval "$(extract_initial_comment_block < "$rbenv_command" | collect_documentation)"
+        [ -n "$summary" ] || continue
+        printf "%s:%s\n" "$command_name" "$summary"
+      done
+    fi
+    [[ $PATH_remain == *:* ]] || break
+    PATH_remain="${PATH_remain#*:}"
   done
   exit 0
 fi