Support a file data iteration interface for large files

Summary: Ref T7149. A couple diffs down the line, this will let us emit chunked files without doing all the work up front or holding the entire file in RAM.

Test Plan:
(Some newlines added for clarity.)

```
$ ./bin/files cat F942
ABCDEFGHIJKLMNOPQRSTUVWXYZ
$ ./bin/files cat F942 --begin 1
BCDEFGHIJKLMNOPQRSTUVWXYZ
$ ./bin/files cat F942 --end 10
ABCDEFGHIJ
$ ./bin/files cat F942 --begin 3 --end 5
DE
$
```

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12071

src/applications/files/management/PhabricatorFilesManagementCatWorkflow.php

@@ -10,6 +10,16 @@ final class PhabricatorFilesManagementCatWorkflow
         pht('Print the contents of a file.'))
       ->setArguments(
         array(
+          array(
+            'name' => 'begin',
+            'param' => 'bytes',
+            'help' => pht('Begin printing at a specific offset.'),
+          ),
+          array(
+            'name' => 'end',
+            'param' => 'bytes',
+            'help' => pht('End printing at a specific offset.'),
+          ),
           array(
             'name'      => 'names',
             'wildcard'  => true,
@@ -31,7 +41,13 @@ final class PhabricatorFilesManagementCatWorkflow
 
     $file = head($this->loadFilesWithNames($names));
 
-    echo $file->loadFileData();
+    $begin = $args->getArg('begin');
+    $end = $args->getArg('end');
+
+    $iterator = $file->getFileDataIterator($begin, $end);
+    foreach ($iterator as $data) {
+      echo $data;
+    }
 
     return 0;
   }

src/applications/files/storage/PhabricatorFile.php

@@ -603,6 +603,31 @@ final class PhabricatorFile extends PhabricatorFileDAO
     return $data;
   }
 
+
+  /**
+   * Return an iterable which emits file content bytes.
+   *
+   * @param int Offset for the start of data.
+   * @param int Offset for the end of data.
+   * @return Iterable Iterable object which emits requested data.
+   */
+  public function getFileDataIterator($begin = null, $end = null) {
+    // The default implementation is trivial and just loads the entire file
+    // upfront.
+    $data = $this->loadFileData();
+
+    if ($begin !== null && $end !== null) {
+      $data = substr($data, $begin, ($end - $begin));
+    } else if ($begin !== null) {
+      $data = substr($data, $begin);
+    } else if ($end !== null) {
+      $data = substr($data, 0, $end);
+    }
+
+    return array($data);
+  }
+
+
   public function getViewURI() {
     if (!$this->getPHID()) {
       throw new Exception(