Persist "Show Changeset" and improve path text selection

Summary:
Ref T13513. Currently:

  - If you click the "Show Changeset" button, your state change doesn't actually get saved on the server.
  - It's hard to select a changeset path name for copy/paste because the "highlight the header" code tends to eat the event.

Instead: persist the former event; make the actual path text not be part of the highlight hitbox.

Test Plan:
  - Clicked "Show Changeset", reloaded, saw changeset visibility persisted.
  - Selected changeset path text without issues.
  - Clicked non-text header area to select/deselect changesets.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21236

webroot/rsrc/js/application/diff/DiffChangeset.js

@@ -886,6 +886,13 @@ JX.install('DiffChangeset', {
     },
 
     _onClickHeader: function(e) {
+      // If the user clicks the actual path name text, don't count this as
+      // a selection action: we want to let them select the path.
+      var path_name = e.getNode('changeset-header-path-name');
+      if (path_name) {
+        return;
+      }
+
       e.prevent();
 
       if (this._isSelected) {
@@ -986,18 +993,16 @@ JX.install('DiffChangeset', {
 
     _onClickShowButton: function(e) {
       e.prevent();
-      this.setVisible(true);
+
+      // We're always showing the changeset, but want to make sure the state
+      // change is persisted on the server.
+      this.toggleVisibility();
     },
 
     isVisible: function() {
       return this._visible;
     },
 
-    _onundo: function(e) {
-      e.kill();
-      this.toggleVisibility();
-    },
-
     getPathView: function() {
       if (!this._pathView) {
         var view = new JX.DiffPathView()