Show change diffs in Phriction

Summary:
This is really rough and needs work (particularly, there's some diff code I
really need to refactor since I sort-of-copy-pasted it) but basically
functional.

Show text changes between diffs and allow users to revert to earlier versions.

Differential's line-oriented diff style isn't ideal for large blocks of text but
I'm betting this is probably good enough in most cases. We can see how bad it is
in practice and then fix it if needbe.

I added a bunch of support for "description" but didn't add the feature in this
diff, I'll either follow up or task it out since it should be a pretty
straightforward change.

Test Plan: Looked at history for several Phriction documents, clicked "previous
change" / "next change", clicked revert buttons.
Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen
CC: aran, hsb, epriestley
Differential Revision: 687

src/infrastructure/diff/engine/PhabricatorDifferenceEngine.php

@@ -0,0 +1,107 @@
+<?php
+
+/*
+ * Copyright 2011 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Utility class which encapsulates some shared behavior between different
+ * applications which render diffs.
+ *
+ * @task config Configuring the Engine
+ * @task diff Generating Diffs
+ */
+final class PhabricatorDifferenceEngine {
+
+  private $ignoreWhitespace;
+
+
+/* -(  Configuring the Engine  )--------------------------------------------- */
+
+
+  /**
+   * If true, ignore whitespace when computing differences.
+   *
+   * @param bool Ignore whitespace?
+   * @return this
+   * @task config
+   */
+  public function setIgnoreWhitespace($ignore_whitespace) {
+    $this->ignoreWhitespace = $ignore_whitespace;
+    return $this;
+  }
+
+
+/* -(  Generating Diffs  )--------------------------------------------------- */
+
+
+  /**
+   * Generate an @{class:DifferentialChangeset} from two raw files. This is
+   * principally useful because you can feed the output to
+   * @{class:DifferentialChangesetParser} in order to render it.
+   *
+   * @param string Entire previous file content.
+   * @param string Entire current file content.
+   * @return @{class:DifferentialChangeset} Synthetic changeset.
+   * @task diff
+   */
+  public function generateChangesetFromFileContent($old, $new) {
+
+    $options = array();
+    if ($this->ignoreWhitespace) {
+      $options[] = '-bw';
+    }
+
+    // Generate diffs with full context.
+    $options[] = '-U65535';
+
+    $old_tmp = new TempFile();
+    $new_tmp = new TempFile();
+
+    Filesystem::writeFile($old_tmp, $old);
+    Filesystem::writeFile($new_tmp, $new);
+    list($err, $diff) = exec_manual(
+      '/usr/bin/diff %Ls %s %s',
+      $options,
+      $old_tmp,
+      $new_tmp);
+
+    if (!strlen($diff)) {
+      // This indicates that the two files are the same (or, possibly, the
+      // same modulo whitespace differences, which is why we can't do this
+      // check trivially before running `diff`). Build a synthetic, changeless
+      // diff so that we can still render the raw, unchanged file instead of
+      // being forced to just say "this file didn't change" since we don't have
+      // the content.
+      $entire_file = explode("\n", $old);
+      foreach ($entire_file as $k => $line) {
+        $entire_file[$k] = ' '.$line;
+      }
+      $len = count($entire_file);
+      $entire_file = implode("\n", $entire_file);
+
+      // This is a bit hacky but the diff parser can handle it.
+      $diff = "--- ignored 9999-99-99\n".
+              "+++ ignored 9999-99-99\n".
+              "@@ -1,{$len} +1,{$len} @@\n".
+              $entire_file."\n";
+    }
+
+    $changes = id(new ArcanistDiffParser())->parseDiff($diff);
+    $diff = DifferentialDiff::newFromRawChanges($changes);
+    return head($diff->getChangesets());
+  }
+
+}