Add linter to check for binaries accidentally added to repository

This should catch mistakes like
a703fbce.

Adds an arcanist-external-json-linter submodule, which should eventually
be mirrored on haskell.org resources.

Test Plan: Validate

Reviewers: thomie, hvr, austin

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1515

arcanist-external-json-linter @ 85ece0f8

+Subproject commit 85ece0f8653e7b1e7de6024d372cfeaf36ab5fa9

.arc-linters/check-binaries.py

+#!/usr/bin/env python
+
+# A linter to warn when binary files are added to the repository
+
+import sys
+import os
+import json
+
+path = sys.argv[1]
+warnings = []
+if os.path.isfile(path):
+    with open(path) as f:
+        if '\0' in f.read(8000):
+            warning = {
+                'severity': 'warning',
+                'message': 'This file appears to be a binary file; does it really belong in the repository?'
+            }
+            warnings.append(warning)
+
+print json.dumps(warnings)

.arcconfig

 {
   "project.name"        : "ghc",
   "repository.callsign" : "GHC",
-  "phabricator.uri"     : "https://phabricator.haskell.org"
+  "phabricator.uri"     : "https://phabricator.haskell.org",
+
+  "load": [
+    ".arc-linters/arcanist-external-json-linter"
+  ]
 }

.arclint

@@ -52,6 +52,10 @@
       "severity": {
         "2": "disabled"
       }
+    },
+    "check-binaries": {
+      "type": "external-json",
+      "external-json.script": "python .arc-linters/check-binaries.py"
     }
   },
 

.gitmodules

@@ -115,3 +115,6 @@
 	path = libffi-tarballs
 	url = ../libffi-tarballs.git
 	ignore = none
+[submodule ".arc-linters/arcanist-external-json-linter"]
+	path = .arc-linters/arcanist-external-json-linter
+	url = https://github.com/bgamari/arcanist-external-json-linter.git