Fix hooks that apply non-utf8 diffs

pre_commit/commands/run.py

@@ -85,9 +85,9 @@ def _run_single_hook(hook, repo, args, write, skips=frozenset()):
     write(get_hook_message(_hook_msg_start(hook, args.verbose), end_len=6))
     sys.stdout.flush()
 
-    diff_before = cmd_output('git', 'diff', retcode=None)
+    diff_before = cmd_output('git', 'diff', retcode=None, encoding=None)
     retcode, stdout, stderr = repo.run_hook(hook, filenames)
-    diff_after = cmd_output('git', 'diff', retcode=None)
+    diff_after = cmd_output('git', 'diff', retcode=None, encoding=None)
 
     # If the hook makes changes, fail the commit
     if diff_before != diff_after:

testing/resources/modified_file_returns_zero_repo/bin/hook.sh

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 
 for f in $@; do
-    echo modified > "$f"
+    # Non UTF-8 bytes
+    echo -e '\x01\x97' > "$f"
     echo "Modified: $f!"
 done