Further improve line grouping in unified views

Summary:
Ref T2009. This tweaks things a bit more to improve consecuitive groups of added and removed lines.

Generally, it gives us "old, old, old, new, new, new" intead of "old, new, old, new, old, new".

Feelin' real good about having unit tests for this stuff.

Test Plan: Unit tests, looked at diffs in web UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11994

src/applications/differential/__tests__/data/generated.diff.one.unshielded

@@ -1,6 +1,6 @@
 N 1 . @generated\n~
 O 2 - \n~
-N 2 + \n~
 O 3 - This is a generated file.\n~
+N 2 + \n~
 N 3 + This is a generated file{(, full of generated code)}.\n~
 N 4 . \n~

src/applications/differential/__tests__/data/groups.diff

@@ -0,0 +1,16 @@
+diff --git a/style b/style
+index a5fc249..18edce4 100644
+--- a/style
++++ b/style
+@@ -57,8 +57,8 @@
+ .dashboard-panel div.phabricator-feed-frame {
+   background: #fff;
+   margin: 0;
+-  border-left: 1px solid {$lightblueborder};
+-  border-right: 1px solid {$lightblueborder};
+-  border-bottom: 1px solid {$blueborder};
++  border-back: 1px solid {$lightblueborder};
++  border-front: 1px solid {$lightblueborder};
++  border-inside: 1px solid {$blueborder};
+   max-width: none;
+ }

src/applications/differential/__tests__/data/groups.diff.one.expect

@@ -0,0 +1,17 @@
+CTYPE 2 1 (unforced)
+style
+style
+-
+X <MISSING-CONTEXT>
+N 57 . .dashboard-panel div.phabricator-feed-frame {\n~
+N 58 .   background: #fff;\n~
+N 59 .   margin: 0;\n~
+O 60 -   border-{(left)}: 1px solid {$lightblueborder};\n~
+O 61 -   border-{(righ)}t: 1px solid {$lightblueborder};\n~
+O 62 -   border-{(bottom)}: 1px solid {$blueborder};\n~
+N 60 +   border-{(back)}: 1px solid {$lightblueborder};\n~
+N 61 +   border-{(fron)}t: 1px solid {$lightblueborder};\n~
+N 62 +   border-{(inside)}: 1px solid {$blueborder};\n~
+N 63 .   max-width: none;\n~
+N 64 . }\n~
+X <MISSING-CONTEXT>

src/applications/differential/__tests__/data/groups.diff.two.expect

@@ -0,0 +1,22 @@
+CTYPE 2 1 (unforced)
+style
+style
+-
+X <MISSING-CONTEXT>
+O 57 . .dashboard-panel div.phabricator-feed-frame {\n~
+N 57 . .dashboard-panel div.phabricator-feed-frame {\n~
+O 58 .   background: #fff;\n~
+N 58 .   background: #fff;\n~
+O 59 .   margin: 0;\n~
+N 59 .   margin: 0;\n~
+O 60 -   border-{(left)}: 1px solid {$lightblueborder};\n~
+N 60 +   border-{(back)}: 1px solid {$lightblueborder};\n~
+O 61 -   border-{(righ)}t: 1px solid {$lightblueborder};\n~
+N 61 +   border-{(fron)}t: 1px solid {$lightblueborder};\n~
+O 62 -   border-{(bottom)}: 1px solid {$blueborder};\n~
+N 62 +   border-{(inside)}: 1px solid {$blueborder};\n~
+O 63 .   max-width: none;\n~
+N 63 .   max-width: none;\n~
+O 64 . }\n~
+N 64 . }\n~
+X <MISSING-CONTEXT>

src/applications/differential/render/DifferentialChangesetRenderer.php

@@ -503,10 +503,6 @@ abstract class DifferentialChangesetRenderer {
           // Ignore it when rendering a one-up diff.
           continue;
         }
-        if ($new_buf) {
-          $out[] = $new_buf;
-          $new_buf = array();
-        }
         $old_buf[] = $primitive;
       } else if ($type == 'new') {
         if ($primitive['line'] === null) {
@@ -514,17 +510,16 @@ abstract class DifferentialChangesetRenderer {
           // old file. Ignore it when rendering a one-up diff.
           continue;
         }
-        if ($old_buf) {
-          $out[] = $old_buf;
-          $old_buf = array();
-        }
         if (!$primitive['htype']) {
           // If this line is the same in both versions of the file, put it in
           // the old line buffer. This makes sure inlines on old, unchanged
           // lines end up in the right place.
 
-          // First, we need to flush the new line buffer if there's anything
-          // in it.
+          // First, we need to flush the line buffers if they're not empty.
+          if ($old_buf) {
+            $out[] = $old_buf;
+            $old_buf = array();
+          }
           if ($new_buf) {
             $out[] = $new_buf;
             $new_buf = array();
@@ -545,15 +540,15 @@ abstract class DifferentialChangesetRenderer {
         if (!$primitive['right']) {
           $out[] = $old_buf;
           $out[] = array($primitive);
-          $out[] = $new_buf;
+          $old_buf = array();
         } else {
           $out[] = $old_buf;
           $out[] = $new_buf;
           $out[] = array($primitive);
+          $old_buf = array();
+          $new_buf = array();
         }
 
-        $old_buf = array();
-        $new_buf = array();
       } else {
         throw new Exception("Unknown primitive type '{$primitive}'!");
       }