Adding -- to the end of git log so can arc land when the branch is

named the same as a file path.

Summary: (In fbcode and www at least, the only places I tried) if a
branch happens to be named the same as a file path (from root) (like if
a branch is called `spec` and there is a directory in `www` called
`spec`) then `arc land` will fail with git complaining that the argument
(to `git log ...`) is ambiguous as it could refer to both a path and a
revision; so this diff adds a `--` to the end so that git knows that
both are revisions and not paths.

Test Plan: Try and land something from www (I did, see D752219) where
the branch is named `spec`.

Reviewed by: epriestley

src/workflow/ArcanistLandWorkflow.php

@@ -353,7 +353,7 @@ EOTEXT
 
     if ($repository_api instanceof ArcanistGitAPI) {
       list($out) = $repository_api->execxLocal(
-        'log --oneline %s %s',
+        'log --oneline %s %s --',
         $this->branch,
         '^'.$this->onto);
     } else if ($repository_api instanceof ArcanistMercurialAPI) {