When a client submits an overlong "sourcePath", truncate it and continue

Summary:
Ref T13385. Currently, if you run `arc diff` in a CWD with more than 255 characters, the workflow fatals against the length of the `sourcePath` database column.

In the long term, removing this property is likely desirable.

For now, truncate long values and continue. This only meaningfully impacts relatively obscure interactive SVN workflows negatively, and even there, "some arc commands are glitchy in very long working directories in SVN" is still better than "arc diff fatals".

Test Plan:
  - Modified `arc` to submit very long source paths.
  - Ran `arc diff`.
    - Before: Fatal when inserting >255 characters into `sourcePath`.
    - After: Path truncated at 255 bytes.

Maniphest Tasks: T13385

Differential Revision: https://secure.phabricator.com/D20727

src/applications/differential/conduit/DifferentialCreateDiffConduitAPIMethod.php

@@ -119,8 +119,11 @@ final class DifferentialCreateDiffConduitAPIMethod
         break;
     }
 
+    $source_path = $request->getValue('sourcePath');
+    $source_path = $this->normalizeSourcePath($source_path);
+
     $diff_data_dict = array(
-      'sourcePath' => $request->getValue('sourcePath'),
+      'sourcePath' => $source_path,
       'sourceMachine' => $request->getValue('sourceMachine'),
       'branch' => $request->getValue('branch'),
       'creationMethod' => $request->getValue('creationMethod'),
@@ -158,4 +161,18 @@ final class DifferentialCreateDiffConduitAPIMethod
     );
   }
 
+  private function normalizeSourcePath($source_path) {
+    // See T13385. This property is probably headed for deletion. Until we get
+    // there, stop errors arising from running "arc diff" in a working copy
+    // with too many characters.
+
+    $max_size = id(new DifferentialDiff())
+      ->getColumnMaximumByteLength('sourcePath');
+
+    return id(new PhutilUTF8StringTruncator())
+      ->setMaximumBytes($max_size)
+      ->setTerminator('')
+      ->truncateString($source_path);
+  }
+
 }