hashchange: Add hashchange.update_browser_history().

This helps us encapsulate the situation where we don't
want to trigger hashchanged(), without having to do it
within sub.js with prevent_once().

static/js/hashchange.js

@@ -168,6 +168,7 @@ function do_hashchange(from_reload) {
 // hash change functionally inert.
 // -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- - -- //
 var state = {
+    is_internal_change: false,
     is_exiting_overlay: false,
     hash_before_overlay: null,
     old_hash: typeof window !== "undefined" ? window.location.hash : "#",
@@ -269,7 +270,29 @@ function hashchanged_overlay(old_hash) {
     state.old_overlay_group = get_hash_group(base);
 }
 
+exports.update_browser_history = function (new_hash) {
+    var old_hash = window.location.hash;
+
+    if (old_hash === new_hash) {
+        // If somebody is calling us with the same hash we already have, it's
+        // probably harmless, and we just ignore it.  But it could be a symptom
+        // of disorganized code that's prone to an infinite loop of repeatedly
+        // assigning the same hash.
+        blueslip.info('ignoring probably-harmless call to update_browser_history: ' + new_hash);
+        return;
+    }
+
+    state.old_hash = old_hash;
+    state.is_internal_change = true;
+    window.location.hash = new_hash;
+};
+
 function hashchanged(from_reload, e) {
+    if (state.is_internal_change) {
+        state.is_internal_change = false;
+        return;
+    }
+
     var old_hash;
     if (e) {
         old_hash = "#" + (e.oldURL || state.old_hash).split(/#/).slice(1).join("");

static/js/stream_edit.js

@@ -4,9 +4,9 @@ var exports = {};
 
 function setup_subscriptions_stream_hash(sub) {
     var id = sub.stream_id;
-    subs.change_state.prevent_once();
 
-    window.location.hash = "#streams" + "/" + id + "/" + hash_util.encodeHashComponent(sub.name);
+    var hash = "#streams" + "/" + id + "/" + hash_util.encodeHashComponent(sub.name);
+    hashchange.update_browser_history(hash);
 }
 
 function settings_for_sub(sub) {

static/js/subs.js

@@ -613,14 +613,9 @@ exports.switch_to_stream_row = function (stream_id) {
 };
 
 exports.change_state = (function () {
-    var prevent_next = false;
 
+    // TODO: flatten this code
     var func = function (hash) {
-        if (prevent_next) {
-            prevent_next = false;
-            return;
-        }
-
         // if there are any arguments the state should be modified.
         if (hash.arguments.length > 0) {
             // if in #streams/new form.
@@ -638,10 +633,6 @@ exports.change_state = (function () {
         }
     };
 
-    func.prevent_once = function () {
-        prevent_next = true;
-    };
-
     return func;
 }());
 
@@ -796,11 +787,7 @@ exports.do_open_create_stream = function () {
 
 exports.open_create_stream = function () {
     exports.do_open_create_stream();
-
+    hashchange.update_browser_history('#streams/new');
-    // this will change the hash which will attempt to retrigger the create
-    // stream code, so we prevent this once.
-    exports.change_state.prevent_once();
-    window.location.hash = "#streams/new";
 };