Make the ArcanistBundle algorithm do what "diff -u" does when hunks are arguably mergeable

Summary:
Ref T13187. See PHI838. If two hunks are separated by 7 lines of context, we can render them as either:

```lang=diff
+ Hunk A
  Context 1
  Context 2
  Context 3
  Context 4
  Context 5
  Context 6
  Context 7
+ Hunk B
```

...or:

```lang=diff
+ Hunk A
  Context 1
  Context 2
  Context 3
@@ +1,2 -3,4 @@
  Context 5
  Context 6
  Context 7
+ Hunk B
```

Since we get the same number of output lines either way and the first one is more human-readable, we picked that one.

However, `diff -u` does the second one. Since human-readability is probably less important than compatibility, change the behavior to be more similar to `diff -u`.

Test Plan: Added unit tests for the edge cases with default parameters (6 context lines, 7 context lines) and made them pass.

Reviewers: amckinley

Maniphest Tasks: T13187

Differential Revision: https://secure.phabricator.com/D19603

src/parser/ArcanistBundle.php

@@ -560,7 +560,16 @@ final class ArcanistBundle extends Phobject {
             }
           }
 
-          if ($jj - $last_change > (($context + 1) * 2)) {
+          // If the context value is "3" and there are 7 unchanged lines
+          // between the two changes, we could either generate one or two hunks
+          // and end up with the same number of output lines. If we generate
+          // one hunk, the middle line will be a line of source. If we generate
+          // two hunks, the middle line will be an "@@ -1,2 +3,4 @@" header.
+
+          // We choose to generate two hunks because this is the behavior of
+          // "diff -u". See PHI838.
+
+          if ($jj - $last_change >= ($context * 2 + 1)) {
             // We definitely aren't going to merge this with the next hunk, so
             // break out of the loop. We'll end the hunk at $break_here.
             break;

src/parser/__tests__/ArcanistBundleTestCase.php

@@ -642,6 +642,24 @@ EODIFF;
         'disjoint-hunks.new')->toUnifiedDiff());
   }
 
+  public function testMergeHunks() {
+    // Hunks should merge if represented by sufficiently few unchanged
+    // lines.
+    $this->assertEqual(
+      $this->loadResource('merge-hunks.diff'),
+      $this->loadOneChangeBundle(
+        'merge-hunks.old',
+        'merge-hunks.new')->toUnifiedDiff());
+
+    // Hunks should not merge if they are separated by too many unchanged
+    // lines.
+    $this->assertEqual(
+      $this->loadResource('no-merge-hunks.diff'),
+      $this->loadOneChangeBundle(
+        'no-merge-hunks.old',
+        'no-merge-hunks.new')->toUnifiedDiff());
+  }
+
   public function testNonlocalTrailingNewline() {
     // Diffs without changes near the end of the file should not generate a
     // bogus, change-free hunk if the file has no trailing newline.

src/parser/__tests__/bundle/merge-hunks.diff

@@ -0,0 +1,19 @@
+Index: file
+===================================================================
+--- file
++++ file
+@@ -2,12 +2,14 @@
+ B
+ C
+ D
++Chocolate
+ E
+ F
+ G
+ H
+ I
+ J
++Caramel
+ K
+ L
+ M

src/parser/__tests__/bundle/merge-hunks.new

@@ -0,0 +1,17 @@
+A
+B
+C
+D
+Chocolate
+E
+F
+G
+H
+I
+J
+Caramel
+K
+L
+M
+N
+O

src/parser/__tests__/bundle/merge-hunks.old

@@ -0,0 +1,15 @@
+A
+B
+C
+D
+E
+F
+G
+H
+I
+J
+K
+L
+M
+N
+O

src/parser/__tests__/bundle/no-merge-hunks.diff

@@ -0,0 +1,20 @@
+Index: file
+===================================================================
+--- file
++++ file
+@@ -2,6 +2,7 @@
+ B
+ C
+ D
++Chocolate
+ E
+ F
+ G
+@@ -9,6 +10,7 @@
+ I
+ J
+ K
++Caramel
+ L
+ M
+ N

src/parser/__tests__/bundle/no-merge-hunks.new

@@ -0,0 +1,17 @@
+A
+B
+C
+D
+Chocolate
+E
+F
+G
+H
+I
+J
+K
+Caramel
+L
+M
+N
+O

src/parser/__tests__/bundle/no-merge-hunks.old

@@ -0,0 +1,15 @@
+A
+B
+C
+D
+E
+F
+G
+H
+I
+J
+K
+L
+M
+N
+O