Define constants for taxonomy hierarchy

Tagging.

I think @acouch is right about #5; that query is checking whether the term has any parents rather than the hierarchy of the vocab. I also noticed one other thing:

+++ b/core/modules/taxonomy/taxonomy.admin.incundefined
@@ -368,7 +368,7 @@ function taxonomy_overview_terms($form, &$form_state, $vocabulary) {
-    if ($vocabulary->hierarchy < 2 && count($tree) > 1) {
+    if ($vocabulary->hierarchy < TAXONOMY_HIERARCHY_MULTIPLE && count($tree) > 1) {

@@ -402,7 +402,7 @@ function taxonomy_overview_terms($form, &$form_state, $vocabulary) {
-  if ($vocabulary->hierarchy < 2 && count($tree) > 1) {
+  if ($vocabulary->hierarchy < TAXONOMY_HIERARCHY_MULTIPLE && count($tree) > 1) {

@@ -702,7 +702,7 @@ function taxonomy_form_term($form, &$form_state, $edit = array(), $vocabulary =
-    '#collapsed' => $vocabulary->hierarchy < 2,
+    '#collapsed' => $vocabulary->hierarchy < TAXONOMY_HIERARCHY_MULTIPLE,

@@ -840,8 +840,8 @@ function taxonomy_form_term_submit($form, &$form_state) {
-  elseif ($current_parent_count > $previous_parent_count && $form['#vocabulary']->hierarchy < 2) {
-    $form['#vocabulary']->hierarchy = $current_parent_count == 1 ? 1 : 2;
+  elseif ($current_parent_count > $previous_parent_count && $form['#vocabulary']->hierarchy < TAXONOMY_HIERARCHY_MULTIPLE) {
+    $form['#vocabulary']->hierarchy = $current_parent_count == 1 ? TAXONOMY_HIERARCHY_SINGLE : TAXONOMY_HIERARCHY_MULTIPLE;

For these, should we avoid relying on the literal value of the disabled and multiple constants being less than the multiple? I think it might be better to check for:

($foo == TAXONOMY_HIERARCHY_DISABLED) || ($foo == TAXONOMY_HIERARCHY_SINGLE)

+++ b/core/modules/taxonomy/taxonomy.admin.incundefined
@@ -368,7 +368,7 @@ function taxonomy_overview_terms($form, &$form_state, $vocabulary) {
-    if ($vocabulary->hierarchy < 2 && count($tree) > 1) {
+    if (($vocabulary->hierarchy == TAXONOMY_HIERARCHY_DISABLED) || ($vocabulary->hierarchy == TAXONOMY_HIERARCHY_SINGLE) && count($tree) > 1) {

@@ -402,7 +402,7 @@ function taxonomy_overview_terms($form, &$form_state, $vocabulary) {
-  if ($vocabulary->hierarchy < 2 && count($tree) > 1) {
+  if (($vocabulary->hierarchy == TAXONOMY_HIERARCHY_DISABLED) || ($vocabulary->hierarchy == TAXONOMY_HIERARCHY_SINGLE) && count($tree) > 1) {

Hm, for these two, let's make the order of operations more explicit:
if (($vocabulary->hierarchy == TAXONOMY_HIERARCHY_DISABLED || $vocabulary->hierarchy == TAXONOMY_HIERARCHY_SINGLE) && count($tree) > 1)

Edit: Actually, how about this (not sure why I didn't think of this earlier):
if (($vocabulary->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE) && (count($tree) > 1))

Thanks @chris.leversuch. Codewise, the patch looks correct to me now. Two minor questions:

+++ b/core/modules/taxonomy/taxonomy.admin.incundefined
@@ -449,7 +449,7 @@ function taxonomy_overview_terms_submit($form, &$form_state) {
+  $hierarchy = TAXONOMY_HIERARCHY_DISABLED; // Update the current hierarchy type as we go.

Sorry that I did not notice this before, but we need to move this comment up to the preceding line since it's now over 80 chars. I'd just reroll/RTBC with that change myself, except...

+++ b/core/modules/taxonomy/taxonomy.moduleundefined
@@ -543,11 +559,11 @@ function taxonomy_check_vocabulary_hierarchy($vocabulary, $changed_term) {
@@ -1210,6 +1226,8 @@ function taxonomy_vocabulary_load_multiple($vids = array(), $conditions = array(

@@ -1210,6 +1226,8 @@ function taxonomy_vocabulary_load_multiple($vids = array(), $conditions = array(
  * @return
  *   The vocabulary object with all of its metadata, if exists, FALSE otherwise.
  *   Results are statically cached.
+ *
+ * @see taxonomy_vocabulary_machine_name_load()
  */
 function taxonomy_vocabulary_load($vid) {
   $vocabularies = taxonomy_vocabulary_load_multiple(array($vid));
@@ -1225,6 +1243,8 @@ function taxonomy_vocabulary_load($vid) {

@@ -1225,6 +1243,8 @@ function taxonomy_vocabulary_load($vid) {
  * @return
  *   The vocabulary object with all of its metadata, if exists, FALSE otherwise.
  *   Results are statically cached.
+ *
+ * @see taxonomy_vocabulary_load()
  */
 function taxonomy_vocabulary_machine_name_load($name) {
   $vocabularies = taxonomy_vocabulary_load_multiple(NULL, array('machine_name' => $name));

These two additions are new to the latest patch in #9. Is that intentional? They don't seem to be related to the issue.

Alright, this looks good now. Thanks @chris.leversuch!

Updated issue summary.