Switch back to zwsp for oncopy line marker

Summary:
Fixes T4759.

Turns out Chrome on windows doesn't really like the word joiner character. We'll switch back to zwsp but make it `position: absolute;` so it doesn't turn into a line break.

Test Plan: Looked at diffs in IE9 and Chrome Windows. Made sure copying still works as expected.

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4759

Differential Revision: https://secure.phabricator.com/D8727

src/applications/diffusion/controller/DiffusionBrowseFileController.php

@@ -789,7 +789,7 @@ final class DiffusionBrowseFileController extends DiffusionBrowseController {
           $anchor_text,
 
           // NOTE: See phabricator-oncopy behavior.
-          "\xE2\x81\xA0",
+          "\xE2\x80\x8B",
 
           // TODO: [HTML] Not ideal.
           phutil_safe_html(str_replace("\t", '  ', $line['data'])),