Fix an unusual nonterminating task graph node

Summary:
Fixes T12114. There were a couple of bugs here:

  - We could draw too many joining lines if a node had a parent with multiple descendants.
  - We could incorrectly ignore columns because of an `unset()`.

I //think// this fixes both things without collateral damage. This whole thing is a little hard to understand/debug and has grown beyond its original scope, so I'll probably rewrite it if there are more issues.

Test Plan:
  - Unit tests.
  - My local repro is clean now:

{F2424920}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12114

Differential Revision: https://secure.phabricator.com/D17211

src/infrastructure/diff/view/PHUIDiffGraphView.php

@@ -50,7 +50,6 @@ final class PHUIDiffGraphView extends Phobject {
 
       $thread_count = $pos;
       for ($n = 0; $n < $thread_count; $n++) {
-
         if (empty($threads[$n])) {
           $line .= ' ';
           continue;
@@ -60,7 +59,7 @@ final class PHUIDiffGraphView extends Phobject {
           if ($found) {
             $line .= ' ';
             $joins[] = $n;
-            unset($threads[$n]);
+            $threads[$n] = false;
           } else {
             $line .= 'o';
             $found = true;
@@ -114,6 +113,7 @@ final class PHUIDiffGraphView extends Phobject {
           if ($thread_commit == $parent) {
             $found = true;
             $splits[] = $idx;
+            break;
           }
         }