Correct various minor diff copy behaviors

Summary:
Ref T12822. Fixes a few things:

  - Firefox selection of weird ranges with an inline between the start and end of the range now works correctly.
  - "Show More Context" rows now render, highlight, and select properly.
  - Prepares for nodes to have copy-text which is different from display-text.
  - Don't do anything too fancy in 1-up/unified mode. We don't copy line numbers after the `content: attr(data-n)` change, but that's as far as we go, because trying to do more than that is kind of weird and not terribly intuitive.

Test Plan:
  - Selected and copied weird ranges in Firefox.
  - Kept an eye on "Show More Context" rows across select and copy operations.
  - Generally poked around in Safari/Firefox/Chrome.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12822

Differential Revision: https://secure.phabricator.com/D20192

webroot/rsrc/js/application/diff/DiffChangesetList.js

@@ -1246,8 +1246,24 @@ JX.install('DiffChangesetList', {
       return changeset.getInlineForRow(inline_row);
     },
 
-    getLineNumberFromHeader: function(th) {
-      return parseInt(th.getAttribute('data-n'));
+    getLineNumberFromHeader: function(node) {
+      var n = parseInt(node.getAttribute('data-n'));
+
+      if (!n) {
+        return null;
+      }
+
+      // If this is a line number that's part of a row showing more context,
+      // we don't want to let users leave inlines here.
+
+      try {
+        JX.DOM.findAbove(node, 'tr', 'context-target');
+        return null;
+      } catch (ex) {
+        // Ignore.
+      }
+
+      return n;
     },
 
     getDisplaySideFromHeader: function(th) {
@@ -1295,7 +1311,7 @@ JX.install('DiffChangesetList', {
     },
 
     _updateRange: function(target, is_out) {
-      // Don't update the range if this "<th />" doesn't correspond to a line
+      // Don't update the range if this target doesn't correspond to a line
       // number. For instance, this may be a dead line number, like the empty
       // line numbers on the left hand side of a newly added file.
       var number = this.getLineNumberFromHeader(target);