Don't require POST to download LFS files from main domain

Summary:
Ref T7789. If you don't have `security.alternate-file-domain` configured, we won't serve binary files over GET.

This is a security measure intended to prevent `<applet src="..." />` attacks and similar, where you upload some "dangerous" binary, include it in another page, and it gets some of the host's permissions because Java/Flash security models are (or were, in the past) goofy.

Allow them to be served over GET if the client is Git LFS. This is safe; these attacks can't add arbitrary HTTP headers.

Test Plan:
Fetched files over GET with and without the LFS header.

```
$ curl -v http://local.phacility.com/file/data/@local/jfht2cxjazi5cmjomfhl/PHID-FILE-sa7mh2pfaocz2adiimeh/netgear_rma.pdf > /dev/null
...
HTTP 302 Redirect
...
```

```
$ curl -v -H 'X-Phabricator-Request-Type: git-lfs' http://localcontent.phacility.com/file/data/@local/jfht2cxjazi5cmjomfhl/PHID-FILE-sa7mh2pfaocz2adiimeh/netgear_rma.pdf > /dev/null
...
HTTP 200 Content
...
```

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D15654

src/applications/diffusion/controller/DiffusionServeController.php

@@ -996,6 +996,7 @@ final class DiffusionServeController extends DiffusionController {
             'href' => $get_uri,
             'header' => array(
               'Authorization' => $no_authorization,
+              'X-Phabricator-Request-Type' => 'git-lfs',
             ),
           );
         } else {