Use a named remote and branches for staging to help git-lfs

Summary:
I'm working on a fairly large monorepo using phabricator and we are
running into a `git-lfs` issue. Every single `arc diff` we have to wait
many seconds while all our lfs files are walked before the push goes through.

I chatted with the `git-lfs` folks in
https://github.com/git-lfs/git-lfs/issues/4076 and they suggested this workflow
change. I tested it on our repo and it does indeed seem to fix the issue. Three
things were change:

1. The remote url is named
2. Use branches instead of tags
3. Do a `git fetch`

(Also, hello after all this time! Hope you're all doing well over at phacility.)

Test Plan:
Before
```
$ arc diff
Linting...
 LINT OKAY  No lint problems.
Running unit tests...
 UNIT OKAY  No unit test failures.
 PUSH STAGING  Pushing changes to staging area...
Uploading LFS objects: 100% (8211/8211), 1.0 GB | 0 B/s, done.
Enumerating objects: 31, done.
Counting objects: 100% (31/31), done.
Delta compression using up to 12 threads
Compressing objects: 100% (21/21), done.
Writing objects: 100% (24/24), 1.77 KiB | 8.00 KiB/s, done.
Total 24 (delta 19), reused 0 (delta 0)
remote: Resolving deltas: 100% (19/19), completed with 6 local objects.
To github.com:robinhoodmarkets/web-staging.git
 * [new tag]             2581578b59b6341c1023377fd8741814e89fcd0c -> phabricator/base/427095
 * [new tag]             7d9be50fbca590f15742d8cc20edd4f082dfbb3b -> phabricator/diff/427095
Updated an existing Differential revision:
        Revision URI: https://phabricator.robinhood.com/D135442

Included changes:
  M       __shared/tools/arcanist-js/src/workflow/RHArcanistDiffWorkflow.php
```

After
```
$ arc diff
Linting...
 LINT OKAY  No lint problems.
Running unit tests...
 UNIT OKAY  No unit test failures.
 PUSH STAGING  Pushing changes to staging area...
Enumerating objects: 15, done.
Counting objects: 100% (15/15), done.
Delta compression using up to 12 threads
Compressing objects: 100% (7/7), done.
Writing objects: 100% (8/8), 638 bytes | 2.00 KiB/s, done.
Total 8 (delta 6), reused 0 (delta 0)
remote: Resolving deltas: 100% (6/6), completed with 6 local objects.
To github.com:robinhoodmarkets/web-staging.git
 * [new branch]          2581578b59b6341c1023377fd8741814e89fcd0c -> phabricator/base/427105
 * [new branch]          274cc3a25eb7856291c1d4c078b28b76af8a43b2 -> phabricator/diff/427105
Updated an existing Differential revision:
        Revision URI: https://phabricator.robinhood.com/D135442

Included changes:
  M       __shared/tools/arcanist-js/src/workflow/RHArcanistDiffWorkflow.php
```

(notice the lack of `Uploading LFS objects`)

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21041

src/workflow/ArcanistDiffWorkflow.php

@@ -2900,6 +2900,9 @@ EOTEXT
       'uri' => $staging_uri,
     );
 
+    list($stdout) = execx('git ls-files -z -- %s', ':(attr:filter=lfs)');
+    $is_lfs = strpos($stdout, "\0") !== false;
+
     // If the base commit is a real commit, we're going to push it. We don't
     // use this, but pushing it to a ref reduces the amount of redundant work
     // that Git does on later pushes by helping it figure out that the remote
@@ -2923,7 +2926,7 @@ EOTEXT
     $refs[] = array(
       'ref' => $diff_tag,
       'type' => 'diff',
-      'commit' => $commit,
+      'commit' => $is_lfs ? $base_commit : $commit,
       'remote' => $remote,
     );
 
@@ -2949,6 +2952,14 @@ EOTEXT
           'use --skip-staging to skip this step.'));
     }
 
+    if ($is_lfs) {
+      $ref = '+'.$commit.':'.$diff_tag;
+      $err = phutil_passthru(
+        'git push -- %s %s',
+        $staging_uri,
+        $ref);
+    }
+
     return $refs;
   }