Fix arc diff --raw with "onto" target properties

Summary:
Currently, `git show | arc diff --raw` and similar doesn't work because we try to figure out what the "Branch: feature (branched from whatever)" value is, which doesn't make sense.

```
$ git show | arc diff --raw --trace
 ARGV  '/Users/epriestley/dev/core/lib/arcanist/bin/../scripts/arcanist.php' 'diff' '--raw' '--trace'
 LOAD  Loaded "phutil" from "/Users/epriestley/dev/core/lib/libphutil/src".
 LOAD  Loaded "arcanist" from "/Users/epriestley/dev/core/lib/arcanist/src".
Config: Reading user configuration file "/Users/epriestley/.arcrc"...
Config: Did not find system configuration at "/etc/arcconfig".
Working Copy: Reading .arcconfig from "/Users/epriestley/dev/core/lib/arcanist/.arcconfig".
Working Copy: Path "/Users/epriestley/dev/core/lib/arcanist" is part of `git` working copy "/Users/epriestley/dev/core/lib/arcanist".
Working Copy: Project root is at "/Users/epriestley/dev/core/lib/arcanist".
Config: Reading local configuration file "/Users/epriestley/dev/core/lib/arcanist/.git/arc/config"...
Loading phutil library from '/Users/epriestley/dev/core/lib/arcanist/src'...
>>> [0] <conduit> conduit.connect() <bytes = 489>
>>> [1] <http> https://secure.phabricator.com/api/conduit.connect
<<< [1] <http> 211,217 us
<<< [0] <conduit> 212,001 us
>>> [2] <event> diff.didCollectChanges <listeners = 0>
<<< [2] <event> 140 us
>>> [3] <event> diff.didBuildMessage <listeners = 0>
<<< [3] <event> 46 us
Reading diff from stdin...
>>> [4] <conduit> differential.creatediff() <bytes = 10,542>
>>> [5] <http> https://secure.phabricator.com/api/differential.creatediff
<<< [5] <http> 120,215 us
<<< [4] <conduit> 120,411 us
>>> [6] <event> diff.wasCreated <listeners = 0>
<<< [6] <event> 41 us
 SKIP STAGING  Raw changes can not be pushed to a staging area.
>>> [7] <conduit> harbormaster.queryautotargets() <bytes = 290>
>>> [8] <http> https://secure.phabricator.com/api/harbormaster.queryautotargets
<<< [8] <http> 217,717 us
<<< [7] <conduit> 217,944 us
>>> [9] <conduit> harbormaster.sendmessage() <bytes = 274>
>>> [10] <http> https://secure.phabricator.com/api/harbormaster.sendmessage
>>> [11] <conduit> harbormaster.sendmessage() <bytes = 274>
>>> [12] <http> https://secure.phabricator.com/api/harbormaster.sendmessage
<<< [10] <http> 123,821 us
<<< [9] <conduit> 134,329 us
<<< [12] <http> 227,580 us
<<< [11] <conduit> 227,787 us

[2016-01-05 10:08:58] EXCEPTION: (Exception) This workflow ('ArcanistDiffWorkflow') requires a Repository API, override requiresRepositoryAPI() to return true. at [<arcanist>/src/workflow/ArcanistWorkflow.php:804]
arcanist(head=master, ref.master=b3e68c9f1793), phutil(head=stable, ref.master=adb8a9c074ba, ref.stable=7b8d38cd2d4e)
  #0 ArcanistWorkflow::getRepositoryAPI() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:2421]
  #1 ArcanistDiffWorkflow::getDiffOntoTargets() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:2411]
  #2 ArcanistDiffWorkflow::updateOntoDiffProperty() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:534]
  #3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:392]
```

Test Plan: Ran `arc diff --raw` in `phabricator/`.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14946

src/workflow/ArcanistDiffWorkflow.php

@@ -2418,6 +2418,10 @@ EOTEXT
   }
 
   private function getDiffOntoTargets() {
+    if ($this->isRawDiffSource()) {
+      return null;
+    }
+
     $api = $this->getRepositoryAPI();
 
     if (!($api instanceof ArcanistGitAPI)) {